[gmx-users] Pressure estimation from NVT

[Mark Abraham]

On 27/09/2011 3:48 AM, Sikandar Mashayak wrote:
> Hi
>
> I am trying to determine pressure of water at given density and 
> temperature using SPC/E water model. To achieve this, I am doing NVT 
> bulk simulations and then using g_energy I get average pressure 
> values. The question I have is how accurate these average pressure 
> predictions are?
>
> e.g. I get average pressure of 185 bar for 300 K and 1 g/cm^3 water 
> density, but theoretical value for it is around 1 bar. What are the 
> possible reasons for this much deviation from theoretical value? Is 
> there anything wrong with my set up? I am using 2141 water molecules 
> in 4*4*4 periodic box, Nose-Hoover thermostat with 0.2ps time constant 
> , MD algorithm with 1fs time-step, LJ with 1.5 nm cut off and PME for 
> electrostatics.

The length of your simulation, and the absence of the equilibration 
period from your data collection are also critical factors in obtaining 
accurate converged averages, but usually neglected when people report 
what might be anomalies... please see 
http://www.gromacs.org/Documentation/Terminology/Pressure

Mark