[Pw_forum] What happens at REALLY large ectuwfc?
Lorenzo: your (ir-)rationale may be more or less justified for separable potentials, but it is certainly wrong for good ol' semilocal ones. SB On Jan 24, 2010, at 9:05 PM, Lorenzo Paulatto wrote: > I'm reposting here a message that I mistakenly sent privately to Brad: > > have a look at the total-energy components: if the one-electron > contributions increases at the expenses of the other terms (mostly > the hartree term) than you have found a ghost in the pseudopotential. > As far as I know, all non-local separable pseudopotentials have ghost > states for large enough values of the curoff. > > I'm not really sure my impression is correct, but I've always had > problems with large enough cutoffs with any pseudopotential I have > tried. My rationale is more or less on this line: the pseudopotential > only have Fourier components up to a certain threshold; plane waves > over that threshold do not feel any direct effect from the ions. As a > consequence this very high frequency plane waves gain nothing by > forming a charge density close to the ions, instead they can just > spread around to minimize the Hartree energy (which decreases) at the > expenses of kinetic energy (which increases). > > cheers > > > -- > Lorenzo Paulatto > SISSA & DEMOCRITOS (Trieste) > phone: +39 040 3787 511 > skype: paulatz > www: http://people.sissa.it/~paulatto/ > > *** save italian brains *** > http://saveitalianbrains.wordpress.com/ > > > > SISSA Webmail https://webmail.sissa.it/ > Powered by Horde http://www.horde.org/ > > > ___ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100124/d9e92b42/attachment-0001.htm
[Pw_forum] What happens at REALLY large ectuwfc?
I'm reposting here a message that I mistakenly sent privately to Brad: have a look at the total-energy components: if the one-electron contributions increases at the expenses of the other terms (mostly the hartree term) than you have found a ghost in the pseudopotential. As far as I know, all non-local separable pseudopotentials have ghost states for large enough values of the curoff. I'm not really sure my impression is correct, but I've always had problems with large enough cutoffs with any pseudopotential I have tried. My rationale is more or less on this line: the pseudopotential only have Fourier components up to a certain threshold; plane waves over that threshold do not feel any direct effect from the ions. As a consequence this very high frequency plane waves gain nothing by forming a charge density close to the ions, instead they can just spread around to minimize the Hartree energy (which decreases) at the expenses of kinetic energy (which increases). cheers -- Lorenzo Paulatto SISSA & DEMOCRITOS (Trieste) phone: +39 040 3787 511 skype: paulatz www: http://people.sissa.it/~paulatto/ *** save italian brains *** http://saveitalianbrains.wordpress.com/ SISSA Webmail https://webmail.sissa.it/ Powered by Horde http://www.horde.org/
[Pw_forum] What happens at REALLY large ectuwfc?
On Jan 23, 2010, at 5:16 , Brad Malone wrote: > So besides the using of a 2000 Ry cutoff for silicon, what else is > wrong here? are you using the latest cvs version? apparently there is a problem with the new symmetrization algorithm that will be fixed ASAP. By the way, fcc Si with k=0 is an unphysical system: it may easily yield funny results P. --- Paolo Giannozzi, Dept of Physics, University of Udine via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
[Pw_forum] elastic constants and internal strain
On Jan 24, 2010, at 4:08 PM, Eduardo Ariel Menendez Proupin wrote:
> Hi all,
Hi Edoardo:
> As far as I understand, PWSCF calculates the stress following Nielsen and
> Martin, PRL 50, 697 (1985), with expressions updated for ultrasoft
> pseudopotentials and maybe other thechnical upgrades. In the formalism of
> Nielsen and Martin, the stress is calculated as a partial derivative of the
> energy versus the strain tensor, hence keeping frozen the atomic coordinates.
to be more precise, scaling all the atomic coordinates homogeneously
> The effect of keeping frosen the atomic fractional coordinates have a large
> effect in the paradigmatic case of the C44 elastic modulus of silicon, where
> there is a large internal strain when the cristal is strained along the [111]
> direction. In the times of 1985, the internal strain contribution to the
> stress was evaluated using more theory and the (I guess, experimental) values
> of the TO phonon frequency.
actually, for that matter, no need to use experimental values.
> In modern times, I would instead relax the atomos in the strained cell, and
> take the value of the stress tensor at the relaxed geometry.
free to do so, but I would still prefer to combine three ingredients that are
easier to obtain, namely the "bare" lattice constan c^0_{44}, the TO frequency,
and the internal strain parameter. These three quantities are the second
derivatives of the energy with respect to: 1) a macroscopic strain, keeping the
atoms frozen at their homoeneously strained positions; 2) the atomic
displacents at zero strain; 3) (1) and (2) mixed derivative. (1) and (2) are
obvious to obtain, (3) is simply the force on atoms linearly indiced by an
applied strain at zero microscopic distortion, or the stress linearly induced
by the displacement of the ions at zero strain.
> Doing so, I think I still lose
> part of the stress, related with the derivative of the atomic positions with
> respect to the strain.
If you do things properly, I do not think you loose anything
> On the other hand, the internal strain can be accounted for, calculating the
> elastic moduli from the second derivative of the energy with respect to the
> strain.
correct
> I tested both methods to obtain the C44 constant of silicon, using the strain
> long the [111] direction. With both methods I obtain the same value of 77.1
> GPa. I expected to obtain different values. What am I missing?
you miss trust in your results. you obtain the same results because they ought
to be the same ...
> I checked that not allowing relaxation, I also obtained the same value for
> the C44(0) using the energy and the stress data. I also checked that the
> constant C11 give consistent values.
no surprise ...
take care,
Stefano
---
Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni
(skype)
La morale est une logique de l'action comme la logique est une morale de la
pens?e - Jean Piaget
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] What happens at REALLY large ectuwfc?
>are you using the latest cvs version? apparently there is a problem >with the new symmetrization algorithm that will be fixed ASAP. The results I posted are from espresso-4.0.5, although I originally saw this problem with espresso-4.1.1 in a different system (AlAs on a 2x2x2 shifted grid). As for what Lorenzo said, it makes sense with what I'm seeing. The energy breakdowns for the 200 Ry and the 2000 Ry cases are shown below: For 200 Ry: !total energy = -14.59208467 Ry Harris-Foulkes estimate = -14.59208467 Ry estimated scf accuracy<3.9E-11 Ry The total energy is the sum of the following terms: one-electron contribution = 5.58227319 Ry hartree contribution = 1.67255719 Ry xc contribution =-5.04795028 Ry ewald contribution= -16.79896478 Ry Fock energy 1 = 0. Ry Fock energy 2 = 0. Ry Half Fock energy 2= 0. Ry - For 2000 Ry: - !total energy = -14.11008918 Ry Harris-Foulkes estimate = -14.11008918 Ry estimated scf accuracy<1.0E-11 Ry The total energy is the sum of the following terms: one-electron contribution = 6.07256114 Ry hartree contribution = 1.58024935 Ry xc contribution =-4.96393489 Ry ewald contribution= -16.79896478 Ry Fock energy 1 = 0. Ry Fock energy 2 = 0. Ry Half Fock energy 2= 0. Ry --- So you can see the one-electron contribution going up and the hartree contribution going down as Lorenzo as argued. Brad -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100124/059131a8/attachment.htm
[Pw_forum] elastic constants and internal strain
Hi all, As far as I understand, PWSCF calculates the stress following Nielsen and Martin, PRL 50, 697 (1985), with expressions updated for ultrasoft pseudopotentials and maybe other thechnical upgrades. In the formalism of Nielsen and Martin, the stress is calculated as a partial derivative of the energy versus the strain tensor, hence keeping frozen the atomic coordinates. The effect of keeping frosen the atomic fractional coordinates have a large effect in the paradigmatic case of the C44 elastic modulus of silicon, where there is a large internal strain when the cristal is strained along the [111] direction. In the times of 1985, the internal strain contribution to the stress was evaluated using more theory and the (I guess, experimental) values of the TO phonon frequency. In modern times, I would instead relax the atomos in the strained cell, and take the value of the stress tensor at the relaxed geometry. Doing so, I think I still lose part of the stress, related with the derivative of the atomic positions with respect to the strain. On the other hand, the internal strain can be accounted for, calculating the elastic moduli from the second derivative of the energy with respect to the strain. I tested both methods to obtain the C44 constant of silicon, using the strain long the [111] direction. With both methods I obtain the same value of 77.1 GPa. I expected to obtain different values. What am I missing? I checked that not allowing relaxation, I also obtained the same value for the C44(0) using the energy and the stress data. I also checked that the constant C11 give consistent values. -- Eduardo Menendez Departamento de Fisica Facultad de Ciencias Universidad de Chile Phone: (56)(2)9787439 URL: http://fisica.ciencias.uchile.cl/~emenendez -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100124/d473cdbb/attachment.htm
[Pw_forum] Could we plot HOMO & LUMO for slab model in QE
Dear all, I want to explain something in very chemical viewpoint. So HOMO and LUMO is very important. I am wondering whether we could plot HOMO, HOMO-1, HOMO-2 or LUMO etc for slab model in QE. thank you for reading best, vega -- == Vega Lew ( weijia liu) Graduate students State Key Laboratory of Materials-oriented Chemical Engineering Nanjing University of Technology, 210009, Nanjing, Jiangsu, China *** Email: vegalew at gmail.com Office: Room A705, Technical Innovation Building, Xinmofan Road 5#, Nanjing, Jiangsu, China *** -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100124/3ac3b16e/attachment.htm
