[gmx-users] NPH simulation, constant enthalpy?

[David van der Spoel]

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.
________________________________________________________________________
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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