# Are "gas phase" calculations the same as "vacuum" calculations?

In both molecular mechanics/dynamics calculations and quantum mechanical calculations, the literature often refers to both gas-phase calculations and calculations done in vacuo. Is there actually a difference between these, or are they the same thing? If same, why would we call something a gas-phase calculation if there's no gas there?

• Note that in-vacuo refers to an object surrounded by a vacuum, so it isn't that nothing is there, it's that nothing is there other than the object (i.e. gas molecule) of interest. Sep 7 '17 at 19:16

• You would start off by building small clusters with explicit solvent: the solute surrounded by some number of $\ce{N2}$ molecules, represented via a force field or some ab initio-derived method. But, what constitutes "gas molecules"? Small clusters are weird models used to reduce computational cost. A true "gas-solute" simulation you're thinking of would be a bulk simulation with dozens or hundreds of solvent molecules that have the density and disordered structure of a gas. Sep 8 '17 at 1:22