[Pw_forum] Bulk modulus of triclinic pentacene

[VARSHA RANI]

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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