You can literally just strain the material along z; i.e. you stretch the lattice constant, keeping the *fractional* coordinates of the atoms the same, and then relax the structure with the z lattice vector constrained (otherwise it will just relax back to the original, unconstrained structure).

Philip James Hasnip

Thank you sir.

I want to calculate the piezoelectric constant which is equal to delta P/delta epsilon (delta P : difference between the polarization of the deformed material and that of the undeformed material, delta e is the applied stress for example 1%). So I want to apply this stress along the z axis.

According to you, I have to calculate the polarization of the undeformed material, then decrease the parameter c by 1% and calculate again its polarization and make the above mentioned ratio????