[Pw_forum] Bulk modulus of triclinic pentacene

[mkondrin]

On 16.12.2015 13:14, VARSHA RANI wrote:
> 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
Yes, you should. For vdW bonded materials the difference between relaxed 
and unrelaxed distorted structures can be very significant.
> time. Also can you suggest me some reference relating elastic constants to bulk modulus for
> triclinic structure.
The keyword is Voigt-Reuss-Hill estimate/average . It is quite general.

Sincerely yours,

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