[Wien] Question regarding volume expansion-contraction after extraction of Li ions
Peter Blaha
pblaha at theochem.tuwien.ac.at
Mon Mar 10 09:01:45 CET 2014
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
> <pblaha at theochem.tuwien.ac.at <mailto:pblaha at 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
> <pblaha at theochem.tuwien.ac.at
> <mailto:pblaha at theochem.tuwien.ac.at>
> <mailto:pblaha at theochem.__tuwien.ac.at
> <mailto:pblaha at 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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> --
>
> P.Blaha
>
> ------------------------------____----------------------------__--__--------------
>
> 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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>
>
> --
> Shamik Chakrabarti
> Senior Research Fellow
> Dept. of Physics & Meteorology
> Material Processing & Solid State Ionics Lab
> IIT Kharagpur
> Kharagpur 721302
> INDIA
>
>
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>
>
> --
>
> P.Blaha
> ------------------------------__------------------------------__--------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at
> <mailto:blaha at theochem.tuwien.ac.at> WWW:
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> <http://info.tuwien.ac.at/theochem/>
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>
>
>
> --
> Shamik Chakrabarti
> Senior Research Fellow
> Dept. of Physics & Meteorology
> Material Processing & Solid State Ionics Lab
> IIT Kharagpur
> Kharagpur 721302
> INDIA
>
>
> _______________________________________________
> Wien mailing list
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--
P.Blaha
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