[gmx-developers] question on PV term

[Michael Shirts]

The PV term computed using pressure control is calculated as:

pv = vol*enerd->term[F_PRES]/PRESFAC;

i.e. the volume times the instantaneous pressure.  It's not clear to
me there is a reason to compute this quantity. In the partition
function, one uses P_ext V, where P is the external pressure that the
system is maintained at, in the same way that when computing E/kT, you
use the external temperature, not the instantaneous average of kinetic
energies.

It could be that there is a use for this quantity, but otherwise, I'd
suggest changing it to

pv = vol*trace(ir->ref_p)/(DIM*PRESFAC)

or the tensor equivalent.