[Pw_forum] Bulk modulus of triclinic pentacene

[VARSHA RANI]

Kondrin, thank you so much for your quick response. But I want to discuss
some more things. I need only bulk modulus not shear modulus. Instead
I need deformation potential also.
So your suggestion of calculating the bulk modulus from the energy difference
between the optimized structure at normal and high pressure can
resolve only the bulk modulus problem.

But can you suggest me something about deformation potential. For my
bulk modulus calculation, initially
 I have used 'vc-relaxd' structure so that I know that my structure is
optimized.

Then after deforming the lattice I ran only 'scf' calculations. should
I still 'relax' after deforming every

time. Also can you suggest me some reference relating elastic
constants to bulk modulus for

triclinic structure.


On Tue, Dec 15, 2015 at 4:53 PM, VARSHA RANI <varsha24_sps at jnu.ac.in> wrote:

> Dear all,
>
> I am trying to calculate the bulk modulus of single crystal Pentacene
> which has a triclinic unit cell with 72 atoms.  Starting from an optimized
> structure, with ibrav = 14 (corresponding to triclinc structure), I
> performed a series of SCF calculations. I changed one of the lattice
> constants by modifying celldm(1), celldm(2) and celldm(3) appropriately. I
> then used ev.x to extract the bulk modulus (with the non-cubic option). I
> also tried to calculate it directly from the parabolic fitting of energy vs
> volume.
>
> From both approaches, I am getting a bulk modulus ~ 350 GPa, which seems
> unrealistic. I have verified that I am able to get the right value of the
> Bulk modulus for Silicon using both approaches.
>
> I was wondering if there is a different way to go about calculating bulk
> modulus values for triclinic structure, or if there is something wrong
> about the way I have set up my input files. I have attached my input files
> for two different configurations, the equilibrium lattice constant and the
> strained value where celldm(1) = a, has been changed by 5%.
>
> I would really appreciate some help with this.
>
> Thanks,
>  Varsha Rani
>
> PhD student
> School of Physical Sciences
> Jawaharlal Nehru University,
> New Delhi, India
>
>
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