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In NVT ensemble runs I have observed large temperature fluctuations. Although total energy is properly conserve (no drift observed), the T fluctuates around a mean value that corresponds to the working T (TEBEG). These fluctuations seems to me extremly large, and are larger for bigger temperatures; for instance, for TEBEG=TEND=100, T fluctuates between ~0 and 200K, and for TEBEG=TEEND=1000, it fluctuates around 0 and 2000K. There are any physical reason linked to Nose thermostat that can explain this? Are my results completly wrong?

P.S.: I have adjust SMASS in order to get a periodic T fluctuation, and trying that it was as small as possible.

as the MD temperature is calculated from the kinetic energy of the ions, there is no general rule of thumb about the acceptable fluctuations. You can fix the final temperature by setting TEEND, but also by doing so, some fluctuations will be left, because of the variations of the sums of the instantaneous kinetic energies of the atoms at the respective time-step