Calculate density expectation as M/<V> instead of M<1/V> about openff-evaluator

[SimonBoothroyd]

It's also easier to then use in the calculation of partial molar volume.

I'm not 100% sure I understand this point - can you please clarify?

The computational cost of computing < V > in addition to < 1 / V > is mostly negligible given that they are (in almost all cases) computed from the same simulation provided they are being computed at the same state.

Given that M/V is output by most simulation packages directly this is the easier quantity to average when computing densities as it avoids the need to compute the molecular weight of the system - while this may be trivial, it's still an extra un-needed couple of steps in the workflow graph (average < V >, computation of M then division as opposed to average < M / V >).

Volume is the more fundamental thermodynamic quantity (dG/dP = V), and the numerical behavior is better (doesn't overweight small values, underweight large values)

While this may be true from a theoretical perspective, I'm not convinced this makes any difference in practice. Can you point to any studies which show there is a significant difference (relative to the typical uncertainty in the estimated density) between the two approaches that justify making the change?