[EXTERNAL] elastic constant for polycrystal

Instead of orthogonal, make the initial box triclinic, with tilt factors
set to zero. Please copy future communications to the LAMMPS list.


Thank You for the reply.I tried to use the the ELASTIC code for FCC-ZrH2 given in LAMMPS and I get negative elastic constants what does it mean?.I am not sure where the mistake is?

Elastic Constant C11all = 531.5063 GPa
Elastic Constant C22all = 531.5063 GPa
Elastic Constant C33all = 531.5063 GPa
Elastic Constant C12all = -174.745 GPa
Elastic Constant C13all = -174.745 GPa
Elastic Constant C23all = -174.745 GPa
Elastic Constant C44all = 65.72833 GPa
Elastic Constant C55all = 65.72833 GPa
Elastic Constant C66all = 65.72833 GPa


As always, I suggest you look at the log file more closely, checking that
the positive and negative deformations gave the same derivative, and that
all the minimization steps (especially the initial one) also converged.
However, given that all your elastic constants appear to satisfy cubic
symmetry to very high precision, I am guessing that this is a correct
result for this potential. The negative value for C12 simply means that
when the crystal is compressed in the y direction, the Pxx component of
the stress is tensile rather than compressive. This is unusual, but it
does not by itself violate the criteria for mechanical stability. From
Problem 7 in Chapter 4 of Kittel Is see that the stability criteria for
cubic crystals are:

C11 > 0, C44 > 0, C11^2-C12^2 > 0, C11+2*C12 > 0

Your crystal satisfies these criteria.