[lammps-users] question about the cell length for Coulomb interaction

Dear all,

I have a system involved with water so Coulomb force should be considered. My simulation box size is 27 x 28 x 200 A, but when I run NVE simulation after a long time NPT, the total energy of the system doesn’t conserve (it goes up with time). What reason it could be? Is it because my simulation box is too small? since the lateral length is only two times larger than Coulomb cutoff. I used the PPPM method to calculate Coulomb force (cutoff 12 A, tolerance 0.0001).

Any suggestions will be highly appreciated!
Regards,

swo

If you run long enough with NVE, any system will slowly
gain energy. Use a thermostat if that's a problem. If it's
going up quickly, then it is probably b/c your system is not
well-equilibrated, or you are using other poor parameters (big
timestep, low PPPM accuracy, etc).

Steve