# [Pw_forum] question relating to crystal optimization

Mojtaba Mirseraji quantum.pwscf at gmail.com
Thu Apr 30 15:31:43 CEST 2015

```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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```