[gmx-users] Re: Enthalpy Confusion

Jeffery Perkins Jeffery.Perkins at ufv.ca
Tue Jun 11 21:57:52 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. 

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

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