[Pw_forum] Bulk modulus of triclinic pentacene

[mkondrin]

On 15.12.2015 15:23, VARSHA RANI 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
>
>
Hi!

If you are only interested in bulk modulus, than you might perform full 
vc-relax optimization at some higher pressures. From energy difference 
between normal and high pressure the bulk modulus can be estimated. If 
you need shear modulus too than you should evaluate all 21 individual 
elastic constant (for triclinic system) by applying some sort of 
deformation and make relaxation ('relax' calculation) of the system in 
the fixed cell. Only 'scf' calculation is not enough, because your 
system is not guaranteed to be in equilibrium.

Hope this helps,

Sincerely yours,
M.V. Kondrin

High Pressure Physics Institute RAS,Troitsk, Russia

>
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