On 2013-06-11 21:57, Jeffery Perkins wrote:
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.
Right, that's an easy one to miss but i don't think that's my problem here.
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
I'm running in NPT and was calcaulating H from ave. P, ave. U and ave. V for
the run
while i understand that this doesn't exactly equal average H it should be
close enough for me at this point, going back and redoing it with P set to
the reference value (which the average hits with small fluctuations)
the resultant H still doesn't line up properly for some example data:
"pV, g_energy" V, m^3 pV, J pV, kJ/mol
442.42 1.46E-26 1.46E-18 884.07
P = 1000 bar = 100,000,000 Pa
and i see that my pV is around 2x g_energies pV, no values are scaled for
the # of items in the box at this point, this is all for moles of the system
i think that's alright though
You should not use pV from g_energy though, as Michael explained, rather
you need ref_p times <V>. This precludes that your system is in
equilibrium of course.
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