you can locate the equilibrium geometry with vc_relax and then manually
apply homogeneous deformation, relaxing the INTERNAL degrees of freedom
and obtaining the elastic constant from the stress strain relationship.
you will need to apply +/- deformations of various strength and extract
the linear term in the variation of the stress tensor. Traceless
deformations are preferable because they don't change the average number
of planewaves. make sure your cutoff is converged for a stress calculation.
If the system has some symmetry this helps reducing the number of
calculations needed.
For instance for cubic symmetry two deformations are enough: an e1
deformation (voigt notation
https://en.wikipedia.org/wiki/Voigt_notation) gives C11 and C12 and e4
gives C44
for a generic crystal you need six deformations (e1,..,e6) to get the 21
independent elastic constants.
stefano
On 29/03/25 15:03, Jonathan Konyi Kakulu wrote:
I have get bulk modulus with ev.x . I want to know how to calculate
the C11, C12 and the C44 constant with PWSCF
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