crystal vibrates widely in NVT and NPT

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1

Hi Stefan,

I used metal unit, so the unit for time is ps.

In my input file, timestep 0.0002, which means my time step is 0.2fs, while Tdamp in fix npt/nvt is 0.02, Pdamp in fix npt is 0.2. This means Tdamp=100dt and Pdamp=1000dt. So it’s not Tdamp or Pdamp causes the problem.

What might cause this vibration? Could you tell me how to reduce it?

Thanks and look forward to your reply.

Rui

If your time step is 0.2 fs there's no way a damping time of 0.2 fs will
work for the thermostat, let alone the barostat. The typical guideline is a
damping time of 100 ? time step size for the thermostat and 1000 ? time
step size for the barostat, though your mileage may vary.
2

Hmm. How large is your system? Fluctuations are natural and, if I remember correctly, they are expected to drop off as 1/sqrt(N) with N the number of particles.
Also, are you sure such vibrations are not to be expected at 600K?