# [Pw_forum] question relating to crystal optimization

Neeraj Goel neeraj.goel179 at gmail.com
Thu Apr 30 15:46:35 CEST 2015

```thank you very much for your suggestion.

regards
neeraj goel

On 4/30/15, Mojtaba Mirseraji <quantum.pwscf at gmail.com> wrote:
> Dear Neeraj
>
> I had this problem, but i could solve it. How?
> you should decrease and increase the "a" and "c/a" simultaneously.
> when the pressure matrix elements in output file of relaxation give
> zero (or very near to zero like 0.01), you are in a successful
> position. After this proceedure, you should decrease or increase the
> "a" and then try relax a series of "c/a". you see minimmum energy of
> e.g.
> a=7.4532187         c/a= 1.011345                       E= -260.893456
>                                     1.012345
> -260.893856
>                                     1.013345
> -260.894556
>                                     1.014345
>  -260.893556
>                                          ...
>             ....
> a=7.4632187         c/a= 1.011345                       E= -260.893656
>                                     1.012345
> -260.894656
>                                     1.013345
> -260.895456
>                                     1.014345
>  -260.894156
>                                          ...
>             ....
> after all these proceedures, you can find the minimmum energy
> for example, in this mentioned example: a=7.4632187, c/a=1.013345 ,
> E=-260.895456
>
> Notice: first, zero the all pressure matrix element
>             second, finding the minimum energy
>
> Regards
>
> Mojtaba Mirseraji
> Ph.D. Candidate
> Theoretical Condensed Matter Physics
> Arak Univ.
> I.R.IRAN
>
>
>
>
> On 4/30/15, Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it> wrote:
>>
>> Dear Neeraj Goel
>> When you optimize the parameters of a non-cubic lattice, you should prefer
>> a
>> vc-relax calculation. Otherwise, you must calculate the total energies on
>> a two-dimension grid (at different a and c values, and centered "near" the
>> expected minimum), and then you fit the minimum energy(a,c) with a given
>> functional form or equation of state (paraboloid, ...).
>> HTH
>>
>> Giuseppe
>>
>> On Thursday, April 30, 2015 10:34:46 AM Neeraj Goel wrote:
>>> i have optimized CuInS2, firstly by varying lattice parameter 'a' with
>>> respect to total energy and find the value of 'a' corresponding to the
>>> total minimum energy, keeping the lattice parameter 'c' constant.
>>> after that i have start varying lattice parameter 'c' and find  the
>>> value of 'c' corresponding to the total minimum energy, this time i
>>> have kept lattice parameter 'a' constant.
>>>           if i calculate in this manner than there will be many
>>> solutions of 'a' corresponding to the each value of 'c'. similar will
>>> be the case with 'c'.
>>>
>>>           so how will i justify the results as i have kept one
>>> parameter constant while varing the other?
>>>
>>>
>>> regards
>>>                                                                   Neeraj
>>> Goel
>>>                                                                Indian
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