[gmx-users] large pressure
Mark Abraham
Mark.Abraham at anu.edu.au
Fri Jul 10 02:05:34 CEST 2009
Shuangxing Dai wrote:
> Hi, all,
> I was wondering why I got large pressure after energy
> minimization. In MD, the pressure and pressure tensor is defined like (
> equation 4.151 in manual):
> P=2*(E_kinetic + W)/(3*V), V is volume and W is Virial.
> I run a simple system, fcc gold with LJ potential to check the pressure
> after energy minimization. I got:
There are no velocities, so whatever is computed here is not the pressure.
> Pressure (bar)
> 8.75158e+03
>
> Steepest Descents converged to Fmax < 1 in 1 steps
> Potential Energy = -1.4870625e+03
> Maximum force = 2.2925207e-05 on atom 1606
> Norm of force = 7.9803776e-06
>
> The force is very small and the energy minimization finished in 1 steps,
> since all the atoms are at equilibrium. So I was wondering why I got
> this huge pressure. And in a lot of simulations, I found huge pressure
> after energy minimization. Even in free energy tutorial on Wiki, I found
> large pressure as larege as 1 GPa.
See http://oldwiki.gromacs.org/index.php/Pressure
> After energy minimization, the forces
> are very small and no velocity, so the pressure should be close to zero.
> Anyone has any idea about this?
I guess it should not be computed.
> PS: Why the forces in traj.trr are all zeros?
EM doesn't write forces.
Mark
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