[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
>              Email: blaha at 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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>
>     --
>
>                                            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
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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
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