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 <[email protected] > <mailto:[email protected]>> 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 > > > > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
