[lammps-users] fix_heat

If heat is subtracted from the system too aggressively so that the group’s kinetic energy goes to zero, LAMMPS halts with an error message.

Probably your lower group has a zero kinetic energy already and you want to subtract energy from it.

Since, the temperature of your system is ZERO to start with, any command with heat extraction is going to give you an error. I think you may want to first equilibrate the whole system at certain non-zero temperature and then incorporate fix heat command.