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>> 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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------------------------------__------------------------------__--------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
Email: bl...@theochem.tuwien.ac.at
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Shamik Chakrabarti
Senior Research Fellow
Dept. of Physics & Meteorology
Material Processing & Solid State Ionics Lab
IIT Kharagpur
Kharagpur 721302
INDIA
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