[gmx-users] NPH simulation, constant enthalpy?

David van der Spoel spoel at xray.bmc.uu.se
Thu Nov 9 08:46:32 CET 2006

Adrien Leygue wrote:
> 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.
You should have constant total energy, not potential (and that is only 
if you have no artifacts due to cutoff etc.)

David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se

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