[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

Dear Professor Giannozzi,

I realized that was just a matter of name.
I got it, thank you,

   ?ric.

On 07/20/2011 02:28 AM, Paolo Giannozzi wrote:
> On Jul 20, 2011, at 15:24 , Eric Germaneau wrote:
>
>> Well, I don't know about "common tangent" concept.
> you should: it is basic thermodynamics. Some time ago
> I wrote something on phase transitions: pag.13-21 of
> http://www.fisica.uniud.it/~giannozz/Corsi/sissa02.pdf
> These lecture notes are not worth much, but given the
> confusion of ideas that emerges from this thread, they
> might be useful
>
> Paolo
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 


College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

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[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/19/2011 10:21 PM, Hongsheng Zhao wrote:
> On 07/20/2011 09:24 PM, Eric Germaneau wrote:
>> Well, I don't know about "common tangent" concept.
> See the page 3 of this notes by Keith Refson:
>
> http://www.castep.org/CASTEP_talks_07/refson2.pdf
Thank you.
>
>
>> You can study structure stability looking at phonon dispersion for instance.
>> An instability could mean that the structure undergoes a phase
>> transition (or that it's not fully relaxed)
> What's the characteristic of the phonon dispersion when phase transition
> occurs?  More hints?
I noticed the reply of Pascal Boulet after I sent my post. He point it 
out very well.
>
>> You can also compute the free energy of your system where phonons will
>> give you the harmonic part (see PRL 64 045123 (2001)
>> ).
>> So you can look at several thing when changing the pressure, among them
>> structure change, phonon, energy, free energy (not that easy).
>> Why not electronic density of sates?
> Again, do you have experience in using DOS for judging phase transition?
>Any hints on the  the characteristic of it when the phase transition
> occurs?
I've no idea, that was just a suggestion.
I suppose some change may occur in the electronic structure ...
But the effect of the pressure on the electronic structure may be less 
significant than the temperature. I don't know.
An expert on /ab initio/ calculation would be able to give you an answer.
>
> Regards
>> Good luck,
>>
>> ?ric.
>>
>> On 07/18/2011 09:46 PM, Amin Torabi wrote:
>>> Thanks Eric and Hongsheng for the discussion!
>>>
>>> Can I also have your comments on this "common tangent" concept in
>>> first order phase transitions? Is it applicable for my case?
>>>
>>>
>>> And how can I use phonon calculations to help me find the phase
>>> transition? I've heard about some "mode softening", but I am not sure
>>> how it can be used in this context? Could you refer me to some article
>>> explaining the theory behind this
>>>
>>> Thanks again
>>> Amin
>>>
>>>
>>>
>>>
>>>
>>> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao
>>> mailto:zhaohscas at yahoo.com.cn>>  wrote:
>>>
>>>  On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>>>  >
>>>  >
>>>  >  On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>>>  >>  On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>>>  >>>  Hey Amin,
>>>  >>>
>>>  >>>  The approach you proposed is ok but you should relax the cell
>>>  as well.
>>>  >>>  By keeping the cell fix the system does not feel the pressure
>>>  you wish
>>>  >>>  to apply.
>>>  >>  Not so clear about the meaning of this sentence, could you
>>>  please give
>>>  >>  some more explanations? ;-(
>>>  >  Under pressure keeping the unit cell fix is useless since the system
>>>  >  won't feel it.
>>>  >  It just makes sense.
>>>  >  So, if you wish to see the effect of the pressure you have to
>>>  perform
>>>  >  vc-relax calculation.
>>>  >  Take it that way, in thermodynamic you have 2 important ensembles,
>>>  >  isochoric and isobaric.
>>>  >  So either the volume is fix and you get out the corresponding
>>>  pressure,
>>>  >  or the pressure is fix and you get out the corresponding volume.
>>>  >  Note that the volume of gas for instance decreases inversely
>>>  >  proportionally to pressure.
>>>  >  The pressure is the intensive variable related to the volume
>>>  (extensive).
>>>  >  You simply can not have both of them constant at the same time.
>>>
>>>  Yes, you're right. I'm so sorry for my previous misleading posts on
>>>  this thread. Thanks again.
>>>
>>>  >  Hope it helps.
>>>  >>
>>>  >>>  You can also change the volume, relax the positions only and
>>>  compute the
>>>  >>>  pressure.
>>>  >>>  The best is to compute the free energy as function of pressure
>>>  ans so
>>>  >>>  the QHA method is the best way to go.
>>>  >>  I think this method cann't do the job of
>>>  structure-searching-and-finding
>>>  >>  for unknown phases when we changing the external pressure.
>>>  >  Yes, it can but in the case of structure-searching it's very
>>>  expensive.
>>>  >  In that case you'd better generate random unit cell parameters,
>>>  random
>>>  >  positions and run vc-relax.
>>>  >  Even this can be expensive since you have to do it a lot of
>>>  times 
>>>  >  Just take a look to this papers, PRL 97 1 (2006)
>>>  >    and JPCM 23
>>>  >  053201 (2011)
>>>  >
>>>  
>>> .
>>>  >  It is the main work of Professor Pickard
>>>  >
>>>  
>>> .
>>>  >  Best.
>>>
>>>  Thanks a lot for this informa

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/20/2011 09:24 PM, Eric Germaneau wrote:
> Well, I don't know about "common tangent" concept.

See the page 3 of this notes by Keith Refson:

http://www.castep.org/CASTEP_talks_07/refson2.pdf


> You can study structure stability looking at phonon dispersion for instance.
> An instability could mean that the structure undergoes a phase
> transition (or that it's not fully relaxed)

What's the characteristic of the phonon dispersion when phase transition 
occurs?  More hints?

> You can also compute the free energy of your system where phonons will
> give you the harmonic part (see PRL 64 045123 (2001)
> ).
> So you can look at several thing when changing the pressure, among them
> structure change, phonon, energy, free energy (not that easy).
> Why not electronic density of sates?

Again, do you have experience in using DOS for judging phase transition? 
  Any hints on the  the characteristic of it when the phase transition 
occurs?

Regards
> Good luck,
>
> ?ric.
>
> On 07/18/2011 09:46 PM, Amin Torabi wrote:
>> Thanks Eric and Hongsheng for the discussion!
>>
>> Can I also have your comments on this "common tangent" concept in
>> first order phase transitions? Is it applicable for my case?
>>
>>
>> And how can I use phonon calculations to help me find the phase
>> transition? I've heard about some "mode softening", but I am not sure
>> how it can be used in this context? Could you refer me to some article
>> explaining the theory behind this
>>
>> Thanks again
>> Amin
>>
>>
>>
>>
>>
>> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao
>> mailto:zhaohscas at yahoo.com.cn>> wrote:
>>
>> On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>> >
>> >
>> > On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>> >> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>> >>> Hey Amin,
>> >>>
>> >>> The approach you proposed is ok but you should relax the cell
>> as well.
>> >>> By keeping the cell fix the system does not feel the pressure
>> you wish
>> >>> to apply.
>> >> Not so clear about the meaning of this sentence, could you
>> please give
>> >> some more explanations? ;-(
>> > Under pressure keeping the unit cell fix is useless since the system
>> > won't feel it.
>> > It just makes sense.
>> > So, if you wish to see the effect of the pressure you have to
>> perform
>> > vc-relax calculation.
>> > Take it that way, in thermodynamic you have 2 important ensembles,
>> > isochoric and isobaric.
>> > So either the volume is fix and you get out the corresponding
>> pressure,
>> > or the pressure is fix and you get out the corresponding volume.
>> > Note that the volume of gas for instance decreases inversely
>> > proportionally to pressure.
>> > The pressure is the intensive variable related to the volume
>> (extensive).
>> > You simply can not have both of them constant at the same time.
>>
>> Yes, you're right. I'm so sorry for my previous misleading posts on
>> this thread. Thanks again.
>>
>> > Hope it helps.
>> >>
>> >>> You can also change the volume, relax the positions only and
>> compute the
>> >>> pressure.
>> >>> The best is to compute the free energy as function of pressure
>> ans so
>> >>> the QHA method is the best way to go.
>> >> I think this method cann't do the job of
>> structure-searching-and-finding
>> >> for unknown phases when we changing the external pressure.
>> > Yes, it can but in the case of structure-searching it's very
>> expensive.
>> > In that case you'd better generate random unit cell parameters,
>> random
>> > positions and run vc-relax.
>> > Even this can be expensive since you have to do it a lot of
>> times 
>> > Just take a look to this papers, PRL 97 1 (2006)
>> >  and JPCM 23
>> > 053201 (2011)
>> >
>> 
>> .
>> > It is the main work of Professor Pickard
>> >
>> 
>> .
>> > Best.
>>
>> Thanks a lot for this information ;-)
>>
>> Regards
>> >
>> >>
>> >> Regards
>> >>
>> >>> Note that at first you can ignore the thermal effect and
>> compute the
>> >>> free energy from a regular phonon calculations.
>> >>> I'm also very interested in such calculations but don't have
>> experiences
>> >>> doing this by mean of first principal calculations.
>>
>>
>>
>> --
>> Hongsheng Zhao > >
>> School of Physics and Electrical Information Science,
>> Ningxia University, Yinchuan 750021, China
>> ___
>> Pw_forum mailing list
>> Pw_foru

[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

Well, I don't know about "common tangent" concept.
You can study structure stability looking at phonon dispersion for instance.
An instability could mean that the structure undergoes a phase 
transition (or that it's not fully relaxed)
You can also compute the free energy of your system where phonons will 
give you the harmonic part (see PRL 64 045123 (2001) 
).
So you can look at several thing when changing the pressure, among them 
structure change, phonon, energy, free energy (not that easy).
Why not electronic density of sates?
Good luck,

   ?ric.

On 07/18/2011 09:46 PM, Amin Torabi wrote:
> Thanks Eric and Hongsheng for the discussion!
>
> Can I also have your comments on this "common tangent" concept in 
> first order phase transitions? Is it applicable for my case?
>
>
> And how can I use phonon calculations to help me find the phase 
> transition? I've heard about some "mode softening", but I am not sure 
> how it can be used in this context? Could you refer me to some article 
> explaining the theory behind this
>
> Thanks again
> Amin
>
>
>
>
>
> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao 
> mailto:zhaohscas at yahoo.com.cn>> wrote:
>
> On 07/19/2011 06:49 PM, Eric Germaneau wrote:
> >
> >
> > On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
> >> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
> >>> Hey Amin,
> >>>
> >>> The approach you proposed is ok but you should relax the cell
> as well.
> >>> By keeping the cell fix the system does not feel the pressure
> you wish
> >>> to apply.
> >> Not so clear about the meaning of this sentence, could you
> please give
> >> some more explanations? ;-(
> > Under pressure keeping the unit cell fix is useless since the system
> > won't feel it.
> > It just makes sense.
> > So, if you wish to see the effect of the pressure you have to
> perform
> > vc-relax calculation.
> > Take it that way, in thermodynamic you have 2 important ensembles,
> > isochoric and isobaric.
> > So either the volume is fix and you get out the corresponding
> pressure,
> > or the pressure is fix and you get out the corresponding volume.
> > Note that the volume of gas for instance decreases inversely
> > proportionally to pressure.
> > The pressure is the intensive variable related to the volume
> (extensive).
> > You simply can not have both of them constant at the same time.
>
> Yes, you're right.  I'm so sorry for my previous misleading posts on
> this thread.  Thanks again.
>
> > Hope it helps.
> >>
> >>> You can also change the volume, relax the positions only and
> compute the
> >>> pressure.
> >>> The best is to compute the free energy as function of pressure
> ans so
> >>> the QHA method is the best way to go.
> >> I think this method cann't do the job of
> structure-searching-and-finding
> >> for unknown phases when we changing the external pressure.
> > Yes, it can but in the case of structure-searching it's very
> expensive.
> > In that case you'd better generate random unit cell parameters,
> random
> > positions and run vc-relax.
> > Even this can be expensive since you have to do it a lot of
> times 
> > Just take a look to this papers, PRL 97 1 (2006)
> >  and JPCM 23
> > 053201 (2011)
> >
> 
> .
> > It is the main work of Professor Pickard
> >
> 
> .
> > Best.
>
> Thanks a lot for this information ;-)
>
> Regards
> >
> >>
> >> Regards
> >>
> >>> Note that at first you can ignore the thermal effect and
> compute the
> >>> free energy from a regular phonon calculations.
> >>> I'm also very interested in such calculations but don't have
> experiences
> >>> doing this by mean of first principal calculations.
>
>
>
> --
> Hongsheng Zhao  >
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org 
> http://www.democritos.it/mailman/listinfo/pw_forum
>
>
>
>
> -- 
> Amin Torabi
> Ph.D. Student
> Chemistry Department
> /The/ University /of/ Western Ontario
> London, On Canada, N6A 5B7
> Phone: 519-661-2111 Ext: 87871
> 
>
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum

-- 
/Be the change you wish to see in the world
/ 

[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

Sorry, I wanted to say opinion (point of view).
My mind gets stuck sometime :-)

On 07/19/2011 04:40 AM, Hongsheng Zhao wrote:
> On 07/19/2011 08:27 PM, Eric Germaneau wrote:
> [snipped]
>> The guy wanted our opening, I got it.
> What's  the meaning of this sentence?  It's so incomprehensible to me.
>
> Regards

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
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[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/18/2011 09:28 PM, Hongsheng Zhao wrote:
> On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>>
>> On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>>> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
 Hey Amin,

 The approach you proposed is ok but you should relax the cell as well.
 By keeping the cell fix the system does not feel the pressure you wish
 to apply.
>>> Not so clear about the meaning of this sentence, could you please give
>>> some more explanations? ;-(
>> Under pressure keeping the unit cell fix is useless since the system
>> won't feel it.
>> It just makes sense.
>> So, if you wish to see the effect of the pressure you have to perform
>> vc-relax calculation.
>> Take it that way, in thermodynamic you have 2 important ensembles,
>> isochoric and isobaric.
>> So either the volume is fix and you get out the corresponding pressure,
>> or the pressure is fix and you get out the corresponding volume.
>> Note that the volume of gas for instance decreases inversely
>> proportionally to pressure.
>> The pressure is the intensive variable related to the volume (extensive).
>> You simply can not have both of them constant at the same time.
> Yes, you're right.  I'm so sorry for my previous misleading posts on
> this thread.  Thanks again.
No worries!
I'm glad to see that this discussion was useful.
Best,

 ?ric.
>
>> Hope it helps.
 You can also change the volume, relax the positions only and compute the
 pressure.
 The best is to compute the free energy as function of pressure ans so
 the QHA method is the best way to go.
>>> I think this method cann't do the job of structure-searching-and-finding
>>> for unknown phases when we changing the external pressure.
>> Yes, it can but in the case of structure-searching it's very expensive.
>> In that case you'd better generate random unit cell parameters, random
>> positions and run vc-relax.
>> Even this can be expensive since you have to do it a lot of times 
>> Just take a look to this papers, PRL 97 1 (2006)
>>   and JPCM 23
>> 053201 (2011)
>> .
>> It is the main work of Professor Pickard
>> .
>> Best.
> Thanks a lot for this information ;-)
>
> Regards
>>> Regards
>>>
 Note that at first you can ignore the thermal effect and compute the
 free energy from a regular phonon calculations.
 I'm also very interested in such calculations but don't have experiences
 doing this by mean of first principal calculations.
>
>

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
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[Pw_forum] Pressure-induced phase transition

[Paolo Giannozzi]

On Jul 20, 2011, at 15:24 , Eric Germaneau wrote:

> Well, I don't know about "common tangent" concept.

you should: it is basic thermodynamics. Some time ago
I wrote something on phase transitions: pag.13-21 of
http://www.fisica.uniud.it/~giannozz/Corsi/sissa02.pdf
These lecture notes are not worth much, but given the
confusion of ideas that emerges from this thread, they
might be useful

Paolo
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 08:27 PM, Eric Germaneau wrote:
[snipped]
> The guy wanted our opening, I got it.

What's  the meaning of this sentence?  It's so incomprehensible to me.

Regards
-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[pascal boulet]

Dear Amin,

Regarding your question:

And how can I use phonon calculations to help me find the phase transition?
I've heard about some "mode softening", but I am not sure how it can be used
in this context?


When pressure changes you may observe that one or several phonon modes
are softening, meaning, their frequencies are decreasing, eventually
becoming imaginary in values. To see this you have to calculate the
phonon dispersion band, that is, the phonon modes over the whole
Brillouin zone (BZ), not just at the gamma point. If for some pressure
you observe a negative (or more correctly imaginary) frequency at a
particular q-point of the BZ, it means that the system undergoes an
instability and is likely to transform into another structure.

This you can see in the following paper: Phys. Rev. B, 78, 100102(R),
2008 in Figure 3b where the phonon softening is observed at the X q-point.


Regards
Pascal




* Anglais - d?tect?
* Anglais
* Fran?ais

* Anglais
* Fran?ais

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 09:46 AM, Amin Torabi wrote:
> Thanks Eric and Hongsheng for the discussion!
>
> Can I also have your comments on this "common tangent" concept in first
> order phase transitions? Is it applicable for my case?

Of course, you can, but be sure this is refer to the E-V plot.

>
>
> And how can I use phonon calculations to help me find the phase
> transition? I've heard about some "mode softening", but I am not sure
> how it can be used in this context? Could you refer me to some article
> explaining the theory behind this

I don't have experience on such thing.  Perhaps others can help.

Regards
>
> Thanks again
> Amin
>
>
>
>
>
> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao  > wrote:
>
> On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>  >
>  >
>  > On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>  >> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>  >>> Hey Amin,
>  >>>
>  >>> The approach you proposed is ok but you should relax the cell
> as well.
>  >>> By keeping the cell fix the system does not feel the pressure
> you wish
>  >>> to apply.
>  >> Not so clear about the meaning of this sentence, could you
> please give
>  >> some more explanations? ;-(
>  > Under pressure keeping the unit cell fix is useless since the system
>  > won't feel it.
>  > It just makes sense.
>  > So, if you wish to see the effect of the pressure you have to perform
>  > vc-relax calculation.
>  > Take it that way, in thermodynamic you have 2 important ensembles,
>  > isochoric and isobaric.
>  > So either the volume is fix and you get out the corresponding
> pressure,
>  > or the pressure is fix and you get out the corresponding volume.
>  > Note that the volume of gas for instance decreases inversely
>  > proportionally to pressure.
>  > The pressure is the intensive variable related to the volume
> (extensive).
>  > You simply can not have both of them constant at the same time.
>
> Yes, you're right.  I'm so sorry for my previous misleading posts on
> this thread.  Thanks again.
>
>  > Hope it helps.
>  >>
>  >>> You can also change the volume, relax the positions only and
> compute the
>  >>> pressure.
>  >>> The best is to compute the free energy as function of pressure
> ans so
>  >>> the QHA method is the best way to go.
>  >> I think this method cann't do the job of
> structure-searching-and-finding
>  >> for unknown phases when we changing the external pressure.
>  > Yes, it can but in the case of structure-searching it's very
> expensive.
>  > In that case you'd better generate random unit cell parameters,
> random
>  > positions and run vc-relax.
>  > Even this can be expensive since you have to do it a lot of times
> 
>  > Just take a look to this papers, PRL 97 1 (2006)
>  >  and JPCM 23
>  > 053201 (2011)
>  >
> 
> .
>  > It is the main work of Professor Pickard
>  >
> 
> .
>  > Best.
>
> Thanks a lot for this information ;-)
>
> Regards
>  >
>  >>
>  >> Regards
>  >>
>  >>> Note that at first you can ignore the thermal effect and
> compute the
>  >>> free energy from a regular phonon calculations.
>  >>> I'm also very interested in such calculations but don't have
> experiences
>  >>> doing this by mean of first principal calculations.
>
>
>
> --
> Hongsheng Zhao mailto:zhaohscas at 
> yahoo.com.cn>>
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org 
> http://www.democritos.it/mailman/listinfo/pw_forum
>
>
>
>
> --
> Amin Torabi
> Ph.D. Student
> Chemistry Department
> /The/ University /of/ Western Ontario
> London, On Canada, N6A 5B7
> Phone: 519-661-2111 Ext: 87871
> 
>
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum


-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>
>
> On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>>> Hey Amin,
>>>
>>> The approach you proposed is ok but you should relax the cell as well.
>>> By keeping the cell fix the system does not feel the pressure you wish
>>> to apply.
>> Not so clear about the meaning of this sentence, could you please give
>> some more explanations? ;-(
> Under pressure keeping the unit cell fix is useless since the system
> won't feel it.
> It just makes sense.
> So, if you wish to see the effect of the pressure you have to perform
> vc-relax calculation.
> Take it that way, in thermodynamic you have 2 important ensembles,
> isochoric and isobaric.
> So either the volume is fix and you get out the corresponding pressure,
> or the pressure is fix and you get out the corresponding volume.
> Note that the volume of gas for instance decreases inversely
> proportionally to pressure.
> The pressure is the intensive variable related to the volume (extensive).
> You simply can not have both of them constant at the same time.

Yes, you're right.  I'm so sorry for my previous misleading posts on 
this thread.  Thanks again.

> Hope it helps.
>>
>>> You can also change the volume, relax the positions only and compute the
>>> pressure.
>>> The best is to compute the free energy as function of pressure ans so
>>> the QHA method is the best way to go.
>> I think this method cann't do the job of structure-searching-and-finding
>> for unknown phases when we changing the external pressure.
> Yes, it can but in the case of structure-searching it's very expensive.
> In that case you'd better generate random unit cell parameters, random
> positions and run vc-relax.
> Even this can be expensive since you have to do it a lot of times 
> Just take a look to this papers, PRL 97 1 (2006)
>  and JPCM 23
> 053201 (2011)
> .
> It is the main work of Professor Pickard
> .
> Best.

Thanks a lot for this information ;-)

Regards
>
>>
>> Regards
>>
>>> Note that at first you can ignore the thermal effect and compute the
>>> free energy from a regular phonon calculations.
>>> I'm also very interested in such calculations but don't have experiences
>>> doing this by mean of first principal calculations.



-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 08:15 AM, Hongsheng Zhao wrote:
> 2-  If you have two or more  initial phases and want to find the phase
> transition pressure among them.  In this case, you can do the relax
> calculation for each phase  by impose external pressure in certain
> range.  And then you can plot the E-V or H-V plots for each phase in the
> same Cartesian system  to find the  phase transition point.

Sorry for my posts on this thread, I'm  confused by some other 
calculatoins with this, say, elastic constans.  Dear Amin, the things 
said by Eric is right.  in any case, for studying the  Pressure-induced 
phase transition, you do always do  vc-relax calculations.  Sorry for my 
misleading disscussions in this thread.

Regards
-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 08:15 AM, Hongsheng Zhao wrote:
> 2-  If you have two or more  initial phases and want to find the phase
> transition pressure among them.  In this case, you can do the relax
> calculation for each phase  by impose external pressure in certain
> range.  And then you can plot the E-V or H-V plots for each phase in the
> same Cartesian system  to find the  phase transition point.

Sorry for my posts on this thread, I'm  confused by some other 
calculatoins with this, say, elastic constans.  Dear Amin, the things 
said by Eric is right.  in any case, for studying the  Pressure-induced 
phase transition, you do always do  vc-relax calculations.  Sorry for my 
misleading disscussions in this thread.

Regards
-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/18/2011 08:15 PM, Hongsheng Zhao wrote:
> On 07/19/2011 12:45 AM, Amin Torabi wrote:
>> Dear Hongsheng,
>>
>> I don't understand why I should use relax instead of vc-relax. As far as
>> I understand, when a system is under pressure, both the atomic positions
>> inside a unit cell, as well as the dimensions of the unit cell changes.
>> So, why should I keep the unit cell parameters fixed if I am interested
>> in finding phase transitions?
> In fact, you can do relax or  vc-relax, it depend on the actual ways
> used by you to study the   Pressure-induced phase transition.  Let me
> give you a more detailed description as follows:
>
> 1-  If you only use one type of initial phase to do this type of study.
>Then you should use the vc-relax when changing the external pressure.
>And then see what's the ultimate unit cell looks like for each
> pressure step.  For convenience, you can also plot the
> binding-energy/formation-energy vs external-pressure to see whether
> there are some abrupt change points - of course, there are many other
> physical properties you can consider to assist you judging the first
> order phase transition, I just give you a example here - on the plot.
>
> 2-  If you have two or more  initial phases and want to find the phase
> transition pressure among them.  In this case, you can do the relax
> calculation for each phase  by impose external pressure in certain
> range.  And then you can plot the E-V or H-V plots for each phase in the
> same Cartesian system  to find the  phase transition point.
>
> Basically, the first method (vc-relax) enable you to find or predict
> some possible phase you don't known in advance.  The second method
> (relax) enable you find the phase transition pressure for known phases.
>
> But, even by using the vc-relax, I don't think it can be served as a
> perfect method for new-phase finding an searching technology.  This is
> because the vc-relax just changing the unit cells ( cell parameters and
> atoms positions ) in a series of localized positions.  So I don't think
> by using this method you can do the job like universal structure
> prediction just as the uspex or similar codes done for you.
They use the free energy and so it's more accurate.
The cell relaxation procedure can give good results (See the work of 
Prof. Pickard).
Anyway, I don't want to debate who is right or wrong. I simply don't 
believe in that.
The guy wanted our opening, I got it.
Best,

?ric.
>
> Regards
>> Thanks
>

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /

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[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 08:15 AM, Hongsheng Zhao wrote:
>
> 1-  If you only use one type of initial phase to do this type of study.
>   Then you should use the vc-relax when changing the external pressure.
>   And then see what's the ultimate unit cell looks like for each
> pressure step.  For convenience, you can also plot the
> binding-energy/formation-energy vs external-pressure to see whether
> there are some abrupt change points - of course, there are many other
> physical properties you can consider to assist you judging the first
> order phase transition, I just give you a example here - on the plot.

Even in this case, you can also do the relax calculation for each 
external pressure step.  Then you plot the 
binding-energy/formation-energy vs external-pressure and find the abrupt 
change point on the plot.   Subsequently, you do the  vc-relax 
calculation for you system by imposing the pressure neighboring to 
abrupt change point and see what's you system should looks like.  By 
this method, you can save some computation times.

Best
-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 12:45 AM, Amin Torabi wrote:
> Dear Hongsheng,
>
> I don't understand why I should use relax instead of vc-relax. As far as
> I understand, when a system is under pressure, both the atomic positions
> inside a unit cell, as well as the dimensions of the unit cell changes.
> So, why should I keep the unit cell parameters fixed if I am interested
> in finding phase transitions?

In fact, you can do relax or  vc-relax, it depend on the actual ways 
used by you to study the   Pressure-induced phase transition.  Let me 
give you a more detailed description as follows:

1-  If you only use one type of initial phase to do this type of study. 
  Then you should use the vc-relax when changing the external pressure. 
  And then see what's the ultimate unit cell looks like for each 
pressure step.  For convenience, you can also plot the 
binding-energy/formation-energy vs external-pressure to see whether 
there are some abrupt change points - of course, there are many other 
physical properties you can consider to assist you judging the first 
order phase transition, I just give you a example here - on the plot.

2-  If you have two or more  initial phases and want to find the phase 
transition pressure among them.  In this case, you can do the relax 
calculation for each phase  by impose external pressure in certain 
range.  And then you can plot the E-V or H-V plots for each phase in the 
same Cartesian system  to find the  phase transition point.

Basically, the first method (vc-relax) enable you to find or predict 
some possible phase you don't known in advance.  The second method 
(relax) enable you find the phase transition pressure for known phases.

But, even by using the vc-relax, I don't think it can be served as a 
perfect method for new-phase finding an searching technology.  This is 
because the vc-relax just changing the unit cells ( cell parameters and 
atoms positions ) in a series of localized positions.  So I don't think 
by using this method you can do the job like universal structure 
prediction just as the uspex or similar codes done for you.

Regards
>
> Thanks


-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>> Hey Amin,
>>
>> The approach you proposed is ok but you should relax the cell as well.
>> By keeping the cell fix the system does not feel the pressure you wish
>> to apply.
> Not so clear about the meaning of this sentence, could you please give
> some more explanations? ;-(
Under pressure keeping the unit cell fix is useless since the system 
won't feel it.
It just makes sense.
So, if you wish to see the effect of the pressure you have to perform 
vc-relax calculation.
Take it that way, in thermodynamic you have 2 important ensembles, 
isochoric and isobaric.
So either the volume is fix and you get out the corresponding pressure, 
or the pressure is fix and you get out the corresponding volume.
Note that the volume of gas for instance decreases inversely 
proportionally to pressure.
The pressure is the intensive variable related to the volume (extensive).
You simply can not have both of them constant at the same time.
Hope it helps.
>
>> You can also change the volume, relax the positions only and compute the
>> pressure.
>> The best is to compute the free energy as function of pressure ans so
>> the QHA method is the best way to go.
> I think this method cann't do the job of structure-searching-and-finding
> for unknown phases when we changing the external pressure.
Yes, it can but in the case of  structure-searching it's very expensive.
In that case you'd better generate random unit cell parameters, random 
positions and run vc-relax.
Even this can be expensive since you have to do it a lot of times 
Just take a look to this papers, PRL 97 1 (2006) 
 and JPCM 23 
053201 (2011) 
.
It is the main work of Professor Pickard 
.
Best.

>
> Regards
>
>> Note that at first you can ignore the thermal effect and compute the
>> free energy from a regular phonon calculations.
>> I'm also very interested in such calculations but don't have experiences
>> doing this by mean of first principal calculations.
>>
>> I hope that helps,
>> Good luck,
>

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /

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[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/19/2011 09:23 AM, Eric Germaneau wrote:
> Hey Amin,
>
> The approach you proposed is ok but you should relax the cell as well.
> By keeping the cell fix the system does not feel the pressure you wish
> to apply.

Not so clear about the meaning of this sentence, could you please give 
some more explanations? ;-(

> You can also change the volume, relax the positions only and compute the
> pressure.
> The best is to compute the free energy as function of pressure ans so
> the QHA method is the best way to go.

I think this method cann't do the job of structure-searching-and-finding 
for unknown phases when we changing the external pressure.

Regards

> Note that at first you can ignore the thermal effect and compute the
> free energy from a regular phonon calculations.
> I'm also very interested in such calculations but don't have experiences
> doing this by mean of first principal calculations.
>
> I hope that helps,
> Good luck,


-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Amin Torabi]

Thanks Eric and Hongsheng for the discussion!

Can I also have your comments on this "common tangent" concept in first
order phase transitions? Is it applicable for my case?


And how can I use phonon calculations to help me find the phase transition?
I've heard about some "mode softening", but I am not sure how it can be used
in this context? Could you refer me to some article explaining the theory
behind this

Thanks again
Amin





On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao wrote:

> On 07/19/2011 06:49 PM, Eric Germaneau wrote:
> >
> >
> > On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
> >> On 07/19/2011 09:23 AM, Eric Germaneau wrote:
> >>> Hey Amin,
> >>>
> >>> The approach you proposed is ok but you should relax the cell as well.
> >>> By keeping the cell fix the system does not feel the pressure you wish
> >>> to apply.
> >> Not so clear about the meaning of this sentence, could you please give
> >> some more explanations? ;-(
> > Under pressure keeping the unit cell fix is useless since the system
> > won't feel it.
> > It just makes sense.
> > So, if you wish to see the effect of the pressure you have to perform
> > vc-relax calculation.
> > Take it that way, in thermodynamic you have 2 important ensembles,
> > isochoric and isobaric.
> > So either the volume is fix and you get out the corresponding pressure,
> > or the pressure is fix and you get out the corresponding volume.
> > Note that the volume of gas for instance decreases inversely
> > proportionally to pressure.
> > The pressure is the intensive variable related to the volume (extensive).
> > You simply can not have both of them constant at the same time.
>
> Yes, you're right.  I'm so sorry for my previous misleading posts on
> this thread.  Thanks again.
>
> > Hope it helps.
> >>
> >>> You can also change the volume, relax the positions only and compute
> the
> >>> pressure.
> >>> The best is to compute the free energy as function of pressure ans so
> >>> the QHA method is the best way to go.
> >> I think this method cann't do the job of structure-searching-and-finding
> >> for unknown phases when we changing the external pressure.
> > Yes, it can but in the case of structure-searching it's very expensive.
> > In that case you'd better generate random unit cell parameters, random
> > positions and run vc-relax.
> > Even this can be expensive since you have to do it a lot of times 
> > Just take a look to this papers, PRL 97 1 (2006)
> >  and JPCM 23
> > 053201 (2011)
> > <
> http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376
> >.
> > It is the main work of Professor Pickard
> > <
> http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html
> >.
> > Best.
>
> Thanks a lot for this information ;-)
>
> Regards
> >
> >>
> >> Regards
> >>
> >>> Note that at first you can ignore the thermal effect and compute the
> >>> free energy from a regular phonon calculations.
> >>> I'm also very interested in such calculations but don't have
> experiences
> >>> doing this by mean of first principal calculations.
>
>
>
> --
> Hongsheng Zhao 
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>



-- 
Amin Torabi
Ph.D. Student
Chemistry Department
*The* University *of* Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871

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[Pw_forum] Pressure-induced phase transition

[Eric Germaneau]

Hey Amin,

The approach you proposed is ok but you should relax the cell as well.
By keeping the cell fix the system does not feel the pressure you wish 
to apply.
You can also change the volume, relax the positions only and compute the 
pressure.
The best is to compute the free energy as function of pressure ans so 
the QHA method is the best way to go.
Note that at first you can ignore the thermal effect and compute the 
free energy from a regular phonon calculations.
I'm also very interested in such calculations but don't have experiences 
doing this by mean of first principal calculations.

I hope that helps,
Good luck,

   ?ric.

On 07/18/2011 04:04 AM, Hongsheng Zhao wrote:
> On 07/18/2011 09:40 AM, Amin Torabi wrote:
>> Dear experts!
>>
>> Forgive me for my naive question... but I am interested in
>> pressure-induced phase transitions of diborane. Could you please have a
>> look at my input file, attached to this message.
>>
>> The following is what I think I should do: (correct me if I'm wrong!)
>>
>> 1. doing separate vc-relax calculation at each pressure
> By using this method, there are some limitations.  You should have known
> in advance the different phases between them the phase transition maybe
> occur when we change the pressure.  Then you can use this method to
> determine the phase transition path.  But I'm not so sure this method
> can predict all of the possible phases for a system when we change the
> external pressure.  To do a more reasonable and complete phase
> transition prediction and even searching for unknown phases, you should
> use some third-party code like uspex for this type of job.
>
> In addition, I think you should do relax, not vc-relax calculation at
> each pressure.  If you do vc-relax, then the pressure imposed to the
> system will be released.  So you won't get the *Pressure-induced* phase
> transition.
>
>> 2. comparing the optimized cell_parameters at each pressure to see when
>> the structure changes
>>
>> I am wondering how I can use the enthalpy or unit cell volume at each
>> pressure to make sure that a phase transition has occurred
> For the other questions, see this thread on this list:  About the method
> to obtain the common tangent for determing the phase transition
> pressure.  HTH.
>
> Regards
>> Thanks in advance
>>
>>
>> --
>> Amin Torabi
>> Ph.D. Student
>> Chemistry Department
>> /The/ University /of/ Western Ontario
>> London, On Canada, N6A 5B7
>> Phone: 519-661-2111 Ext: 87871
>> 
>>
>>
>>
>> ___
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://www.democritos.it/mailman/listinfo/pw_forum
>

-- 
/Be the change you wish to see in the world
/ --- Mahatma Gandhi ---

Dr. ?ric Germaneau 

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /

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[Pw_forum] Pressure-induced phase transition

[Hongsheng Zhao]

On 07/18/2011 09:40 AM, Amin Torabi wrote:
> Dear experts!
>
> Forgive me for my naive question... but I am interested in
> pressure-induced phase transitions of diborane. Could you please have a
> look at my input file, attached to this message.
>
> The following is what I think I should do: (correct me if I'm wrong!)
>
> 1. doing separate vc-relax calculation at each pressure

By using this method, there are some limitations.  You should have known 
in advance the different phases between them the phase transition maybe 
occur when we change the pressure.  Then you can use this method to 
determine the phase transition path.  But I'm not so sure this method 
can predict all of the possible phases for a system when we change the 
external pressure.  To do a more reasonable and complete phase 
transition prediction and even searching for unknown phases, you should 
use some third-party code like uspex for this type of job.

In addition, I think you should do relax, not vc-relax calculation at 
each pressure.  If you do vc-relax, then the pressure imposed to the 
system will be released.  So you won't get the *Pressure-induced* phase 
transition.

> 2. comparing the optimized cell_parameters at each pressure to see when
> the structure changes
>
> I am wondering how I can use the enthalpy or unit cell volume at each
> pressure to make sure that a phase transition has occurred

For the other questions, see this thread on this list:  About the method 
to obtain the common tangent for determing the phase transition 
pressure.  HTH.

Regards
>
> Thanks in advance
>
>
> --
> Amin Torabi
> Ph.D. Student
> Chemistry Department
> /The/ University /of/ Western Ontario
> London, On Canada, N6A 5B7
> Phone: 519-661-2111 Ext: 87871
> 
>
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum


-- 
Hongsheng Zhao 
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

[Pw_forum] Pressure-induced phase transition

[Amin Torabi]

Dear Hongsheng,

I don't understand why I should use relax instead of vc-relax. As far as I
understand, when a system is under pressure, both the atomic positions
inside a unit cell, as well as the dimensions of the unit cell changes. So,
why should I keep the unit cell parameters fixed if I am interested in
finding phase transitions?

Thanks

On Mon, Jul 18, 2011 at 4:04 AM, Hongsheng Zhao wrote:

> On 07/18/2011 09:40 AM, Amin Torabi wrote:
> > Dear experts!
> >
> > Forgive me for my naive question... but I am interested in
> > pressure-induced phase transitions of diborane. Could you please have a
> > look at my input file, attached to this message.
> >
> > The following is what I think I should do: (correct me if I'm wrong!)
> >
> > 1. doing separate vc-relax calculation at each pressure
>
> By using this method, there are some limitations.  You should have known
> in advance the different phases between them the phase transition maybe
> occur when we change the pressure.  Then you can use this method to
> determine the phase transition path.  But I'm not so sure this method
> can predict all of the possible phases for a system when we change the
> external pressure.  To do a more reasonable and complete phase
> transition prediction and even searching for unknown phases, you should
> use some third-party code like uspex for this type of job.
>
> In addition, I think you should do relax, not vc-relax calculation at
> each pressure.  If you do vc-relax, then the pressure imposed to the
> system will be released.  So you won't get the *Pressure-induced* phase
> transition.
>
> > 2. comparing the optimized cell_parameters at each pressure to see when
> > the structure changes
> >
> > I am wondering how I can use the enthalpy or unit cell volume at each
> > pressure to make sure that a phase transition has occurred
>
> For the other questions, see this thread on this list:  About the method
> to obtain the common tangent for determing the phase transition
> pressure.  HTH.
>
> Regards
> >
> > Thanks in advance
> >
> >
> > --
> > Amin Torabi
> > Ph.D. Student
> > Chemistry Department
> > /The/ University /of/ Western Ontario
> > London, On Canada, N6A 5B7
> > Phone: 519-661-2111 Ext: 87871
> > 
> >
> >
> >
> > ___
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
>
>
> --
> Hongsheng Zhao 
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>



-- 
Amin Torabi
Ph.D. Student
Chemistry Department
*The* University *of* Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871

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[Pw_forum] Pressure-induced phase transition

[Amin Torabi]

Dear experts!

Forgive me for my naive question... but I am interested in pressure-induced
phase transitions of diborane. Could you please have a look at my input
file, attached to this message.

The following is what I think I should do: (correct me if I'm wrong!)

1. doing separate vc-relax calculation at each pressure
2. comparing the optimized cell_parameters at each pressure to see when the
structure changes

I am wondering how I can use the enthalpy or unit cell volume at each
pressure to make sure that a phase transition has occurred

Thanks in advance


-- 
Amin Torabi
Ph.D. Student
Chemistry Department
*The* University *of* Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871

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[Pw_forum] Pressure-induced phase transition

[Amin Torabi]

Dear experts!

Forgive me for my naive question... but I am interested in pressure-induced
phase transitions of diborane. Could you please have a look at my input
file, attached to this message.

The following is what I think I should do: (correct me if I'm wrong!)

1. doing separate vc-relax calculation at each pressure
2. comparing the optimized cell_parameters at each pressure to see when the
structure changes

I am wondering how I can use the enthalpy or unit cell volume at each
pressure to make sure that a phase transition has occurred

Thanks in advance


-- 
Amin Torabi
Ph.D. Student
Chemistry Department
*The* University *of* Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871

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