How large are the forces during the vol,ca,ba optimization (before you
have done a final optimization) ?
I do not know anything about the specific material or details of your
results or the VASP calculations. How should I guide you ?
I only know that technical one must optimize ALWAYS simultaneously these
parameters. Of course, in certain cases, there could be negligible
effects ...
On 03/10/2014 08:29 AM, shamik chakrabarti wrote:
Sir,
The forces in our fully optimized structure is less than 3mRy/a.u.
per atom...also we have done coordinate optimization by taking fully
optimized lattice parameters......
Yes...we have not done the simultaneous optimization of lattice
parameters and atomic coordinates...while we have done all the
optimizations in steps: volume, c/a, b/a, atomic coordinates.
Also, there are several peer reviewed papers on calculation on the same
material with VASP and they have all achieved volume expansion.......
Sir...is this mean ...from this discussion..it is coming out that we
should try with simultaneous optimization of lattice parameters and
atomic coordinates for final verification....?
but as the energy is mainly depends on volume...& not on the lattice
parameters...I really have doubt whether this method would change the
trend of volume contraction towards expansion.
On Mon, Mar 10, 2014 at 12:46 PM, Peter Blaha
<pbl...@theochem.tuwien.ac.at <mailto:pbl...@theochem.tuwien.ac.at>> wrote:
You cannot expect that a,b,c is correct when you fix atomic
positions (which are allowed to vary according to symmetry). But
without doing these calculations (or at least checking how large are
the forces in your "best" structure) one cannot say anything.
On the other hand: I do not know if the VASP calculation is correct
either ...
On 03/10/2014 08:06 AM, shamik chakrabarti wrote:
Dear Prof. Blaha,
*we have also used the option 6 for optimization of delithiated
compound...but in that case also we have obtained decrements in unit
cell volume with Li ion extraction.*
We have also used GGA, and GGA+U for lattice parameter
optimization. As,
usual with GGA+U approach we have obtained higher lattice parameters
than the case with GGA. However, decrements of unit cell volume
with Li
extraction has been apparent for both the cases.
Yes it could be the fact that we may need to modify the script for
simultaneous optimization of coordinates with lattice parameter
optimization....whether it could lead to the
solution?....whether the
decrements of unit cell volume with Li delithiation can be
avoided & we
actually will get an increment by this method?.....
Most importantly, although we are getting a volume reduction with Li
de-intercalation (opposite to VASP)...we are getting very well
matched
values of density of states and voltage (from total energy
consideration).
Looking forward to your further discussion which may shed some light
over our problems.
with regards,
On Mon, Mar 10, 2014 at 12:16 PM, Peter Blaha
<pbl...@theochem.tuwien.ac.at
<mailto:pbl...@theochem.tuwien.ac.at>
<mailto:pblaha@theochem.__tuwien.ac.at
<mailto:pbl...@theochem.tuwien.ac.at>>> wrote:
The options "optimize vol, c/a or b/a" are "special"
options for
certain properties, but for sure a sequential optimization
along
these lines does NOT give the minimum a,b,c.
In your case it seems you should use option [6], vary a,b,c.
Furthermore: it is vital, that you optimize internal
coordinates
(MSR1a or min_lapw) for each of these steps (modify the
resulting
script).
When you are talking about Fe2+,3+; you have to think
whether GGA+U
is necessary ??
On 03/10/2014 06:32 AM, shamik chakrabarti wrote:
Dear wien2k users,
We have simulated structural, elcetronic
properties and
cathode
characteristics (in terms of voltage) of a Li based
material.
When we
extract some Li ions from the parent compounds and
optimize its
volume
and lattice parameters, we have obtained a decrements
in volume.
This
decrements has been followed from the concern that Fe+2
ion get
oxidized
in Fe+3 ions in the delithiated compounds & hence there
is an
decrements
in Fe-O bond length in comparison to the parent
(lithiated) one.
However, simulation report on the same compound,
computed in
VASP, has
shown that there is actually an expansion of volume
after Li
extraction.
This expansion occurs as when Li ions are taken out
from the parent
materials there is a decrements of attraction between
two layers
connected with Li ions previously. At least this is the
explanation
given by earlier researchers.
we optimize the material and its Li extracted
counterpart by
following
the methods:
(1) volume optimization by keeping a:b:c constant, (2)
optimizing c/a
ratio and (3) optimizing b/a ratio
we have taken lattice parameters after (3) rd operation
(optimization)
and calculated DOS and voltage.
we have also seen....that after 3rd optimization
(1) there is an increment in b/a ratio in the
delithiated part
than in
the lithiated part in agreement with VASP
(2) However, unit cell volume of the de-lithiated part
is less
than the
lithiated compound due to more dominant reduction of
Fe-O bond
lengths
in comparison to the increment of layer-layer
separation due to
Li ion
extractions in our calculation.
Now my queries are:
(1) Is the reduction of Fe-O bond length after Li
extraction is more
than the decrements of layer-layer separation in FPLAPW
calculation?...&
that's why we have achieved decrements of volume instead of
increments
as obtained in VASP?
(2) Whether this difference came out from some inherent
fact of
FPLAPW
and PAW (VASP) approach?
(3) Whether this is the limitation of the wien2k code
itself?
(4) Or, whether our methodology of calculation by first
volume
optimization & then further optimizing c/a and b/a
ration....is
actually
a wrong path...& we should choose some other way for
lattice
parameter
optimization after Li extraction.
Sorry for such a long email. However, the answer of these
queries are
very necessary for our further research and I believe, this
discussion
will help the community as a whole.
Thanks in advance,
with regards,
--
Shamik Chakrabarti
Senior Research Fellow
Dept. of Physics & Meteorology
Material Processing & Solid State Ionics Lab
IIT Kharagpur
Kharagpur 721302
INDIA
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------------------------------____----------------------------__--__--------------
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------------------------------__------------------------------__--------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
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IIT Kharagpur
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