[gmx-users] Pressure average problem

[nishap.patel at utoronto.ca]

Hello,

     I am simulating a box of water (TIP3P ) in a box of 3nm*3*3nm.My  
simulations are 100ns each. I am using different cut-off. i.e
vdw-type                 = switch
rvdw-switch              = 0.8
rvdw                     = 1.0, 1.2 and 1.3

and these are my parameters for Pressure coupling":
Pcoupl                   = Parrinello-Rahman
Pcoupltype               = isotropic
tau_p                    = 2.0
compressibility          = 4.5e-5
ref_p                    = 1.0

Now after my simulations, when I use g_energy to extract average  
pressure I get different values for all three cut-offs.
For the cut-off of 1.0nm, the average value doesn't converge to 1.0atm  
but it is rather ~2.1atm, for 1.2cut off the avg. pressure is ~1.7 and  
for 1.3cut off avg. pressure is ~1.03. I don't understand why the  
pressure is not converging to 1.0atm for the cut-off of 1.0nm.. All  
the parameters are exactly the same, the only difference between these  
simulations are the cut-off values. Could anyone please give me some  
insight on how the pressure coupling works and if this behaviour seems  
normal?


Thanks.
Nisha Patel