[gmx-users] Huge fluctuations for pressure coupling

[Bob Johnson]

Hello everyone,
I have a 20 base single stranded DNA with counterions that is hydrated with
about 9000 water molecules. My pressure coupling input looks like this:

pcoupl              =  Parrinello-Rahman
pcoupltype          =  isotropic
ref_p               =  1.01
compressibility     =  4.5e-5
tau_p               =  1.0

However, I get huge pressure fluctuations (RMS fluctuation of 1.71704e+02) and
it seems that the coupling doesn't even enforce the correct pressure (average
pressure of 1.68282e-01). I didn't see anyone in the mailing list that had a
similar problem. Does anyone know what's going on?
Thanks,
Bob