Instead of orthogonal, make the initial box triclinic, with tilt factors
set to zero. Please copy future communications to the LAMMPS list.
Hi
Thank You for the reply.I tried to use the the ELASTIC code for FCCZrH2 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

Thanks
Ravi
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, C11^2C12^2 > 0, C11+2*C12 > 0
Your crystal satisfies these criteria.
Aidan