system's center of mass drifting during BOMD NVT simulation

[lar... at lbl.gov]

Dear all,

I have been studying water systems of different sizes for a while now (I 
have attached here an example input file ). In these simulations I 
consistently observe the center of mass of the system drifting (i.e. the 
whole system is moving).

I'm running BOMD in the NVT ensemble using a Nose-Hoover massive thermostat 
chain. My best guess to explain the drift of the system is that the random 
velocities imposed by the thermostat (maybe during initialization) are not 
canceling out exactly. Is there a way of constraining the momentum of the 
system's center of mass to get rid of this issue?

I am also observing a sort of estrange long term behavior in the 
temperature where the temperature vs. time curve exhibits a long-wave 
fluctuation (I have attached a plot here, x-axis is number of steps and 
y-axis is temperature in K). I'm afraid this could be an artifact of the 
thermostat.

If this is not an artifact of the thermostat I am not quite sure what could 
be causing it, any other ideas?

Thanks for your help!
Luis
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