[gmx-users] NPH simulation, constant enthalpy?

[Adrien Leygue]

Dear Gromacs users,

I have been trying to run an NPH (constant pressure and enthalpy)
simulation of a LJ fluid (as a first test) with GROMACS. From what I
understand (from the GROMACS manual and references therein), one can
do that by considering a Parrinello-Rahman pressure coupling (and no
thermostat).

However, when I compute the enthalpy a posteriori using g_energy, it
is far from constant and actually fluctuates a lot. But from the Nosé
& Klein paper (cited in the manual, in the section on pressure
coupling) it seems that I really should have a constant enthalpy,
except for small fluctuations (associated to the degrees of freedom of
the barostat, I believe).

So I'm probably missing something.

If anyone has ever tried similar simulations with Gromacs, I'd very
much like any input or advice you might have on what I might be doing
wrong.

Thanks a lot.

Adrien.

-- 
Adrien Leygue

Department of Materials Science and Engineering
School of Chemical Engineering
National Technical University of Athens
9 Heroon Polytechniou Street
Zografou Campus, Athens 157 80,
GREECE
Tel/Fax: +30210 772-3112