[gmx-users] Re: Enthalpy Confusion

Michael Shirts mrshirts at gmail.com
Tue Jun 11 21:15:50 CEST 2013

If you are computing enthaply in the NPT ensemble, P is constant, and
is the applied pressure.

The "pressure" quantity calculated from the KE and the virial is not
the pressure.  It is a quantity that when averaged over time is equal
the pressure.  Only the average is meaningful macroscopically.

If you are computing enthalpy in another ensemble (which is possible,
though it may be harder to interpret) then you would use the average
pressure from this quantity.

On Tue, Jun 11, 2013 at 3:08 PM, Jeffery Perkins <Jeffery.Perkins at ufv.ca> wrote:
> that's what i thought, and what i tried to do, my pressure is a bit higher
> then that, we want a Lennard-Jones liquid so it's running at 1000+ bar, and
> while I agree that gromacs is giving H as Etot + pV it appears that when i
> calculate pV i get a different value from what g_energy returns for it I did
> p in Pa, V in m^3, as the whole box, to get J of energy, and then multiply
> by 6.02E23 to get J/mol of my box, and then convert down to kJ/mol to be the
> same units as g_energy.
> When you say the applied pressure you mean that p = ref_p? or the calculated
> pressure from the virial and Ke?
> Thanks for the help,
> Jeffery
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