Pressure relaxation during minimization

Dear LAMMPS Users,

I am trying to replicate the system described in however I am having trouble obtaining the correct elastic constants.

It is PbTe with NaCl-type structure.
Potential includes Coulomb part, Morse part between Pb-Te and Te-Te pairs, and cosine/squared interaction for Te-Pb-Te angle.

pair_style hybrid/overlay morse/smooth/linear 8 coul/cut 20.0
pair_coeff * * coul/cut 20.0
pair_coeff 1 2 morse/smooth/linear 0.465 0.863 3.68 7
pair_coeff 2 2 morse/smooth/linear 0.494 1.51 4.22 8

angle_style cosine/squared
angle_coeff 1 0.340 90.0

I am setting the system via read_restart command from custom made file (PbTe with NaCl-type structure and angles: twelve 90 degree angles Te-Pb-Te per every Pb atom).

When I try to calculate the elastic constants, the minimization (fix 3 all box/relax aniso 0.0) does not reach pressure close to 0 (i.e. it is usually lower than -100 000 bars and does not depend on initial lattice parameter), despite zeroing forces. Also, the minimization itself sometimes converges to unphysical energy values (e.g. 1e+200 eV).

I am using the ELASTIC example.
I have tried different cutoff distances, initial lattice parameters, types of minimization (cg, sd) however without success.
I would be very gratefull if someone could tell me the possible reasons for such behaviour.


it is difficult to make any serious comments with so little information.
most of the time when people get "funny" results, there are either a
bad geometry, bad parameters/settings in the input or bad force field
when taking data from a paper, there are sometimes typos or units are
not converted as needed.