On the Theory of Simple Liquids

A theory of interacting molecules consists of a quantum-mechanical part giving the details of the interaction and a statistical part considering the molecular configurations for a given interaction. The first task, a theory of intermolecular forces, is still in its early stages, especially for molecular distances of interest in the liquid state. In practice, we are dealing with empirical pair potentials, at best adjusted to the theoretical long distance behaviourl. Several important details of the empirical potentials are still uncertain. Therefore, it is impossible to test intermolecular interaction and statistical theory separately by comparing theoretical predictions and experimental behaviour. In this situation, it is very fortunate that computer experiments permit the construction of model assemblies of particles with given interactions. Without going into the details of the Monte Carlo method or the method of molecular dynamics’, it should be said that the investigation of 500-1000 particles over sufficiently long times leads to results for pressure and energy which are indistinguishable from the values of much larger assemblies. This is brought about mainly by the use of the so-called periodic boundary conditions. The problem remains of how to arrive at values for entropy or chemical potential. For this it is necessary to construct proper reference states, which are continuously connected to the state in question, so that an integration over reciprocal volume or reciprocal