Interaction site fluids

RECENT PROGRESS IN THE STATISTICAL MECHANICS OF INTERACTION SITE FLUIDS... 

We have chosen to focus almost entirely on the theoretical techniques themselves and on applications that test the accuracy of these techniques. Thus, we will generally refer to the rather large literature on the study of interaction site fluids via Monte Carlo and molecular dynamics simulations only as it impinges directly upon the theoretical developments. We have also taken advantage of the availability of review articles by Chandler and Rossky in limiting our discussion of applications such as the calculation of specific liquid structures and chemical equilibrium, which were described in detail in those articles. 

In this section, we review some of the important formal results in the statistical mechanics of interaction site fluids. These results provide the basis for many of the approximate theories that will be described in Section III, and the calculation of correlation functions to describe the microscopic structure of fluids. We begin with a short review of the theory of the pair correlation function based upon cluster expansions. Although this material is featured in a number of other review articles, we have chosen to include a short account here so that the present article can be reasonably self-contained. Cluster expansion techniques have played an important part in the development of theories of interaction site fluids, and in order to fully grasp the significance of these developments, it is necessary to make contact with the results derived earlier for simple fluids. We will first describe the general cluster expansion theory for fluids, which is directly applicable to rigid nonspherical molecules by a simple addition of orientational coordinates. Next we will focus on the site-site correlation functions and describe the interaction site cluster expansion. After this, we review the calculation of thermodynamic properties from the correlation functions, and then we consider the calculation of the dielectric constant and the Kirkwood orientational correlation parameters. 

As we mentioned in the opening paragraph, thermodynamic perturbation theory has been used in two contexts in applications to interaction site fluids. In this section, we will describe efforts to treat the thermodynamics and structure of interaction site fluids in terms of a perturbation expansion where the reference system is a fluid in which the intermolecular forces are spherically symmetric. In developing thermodynamic perturbation theories, it is generally necessary to choose both a reference system and a function for describing the path between the reference fluid and the fluid of interest. The latter choice is usually made between the pair potential and its Boltzmann factor. Thus one writes either... 

The most difficult and least satisfactory part of the theories is the calculation of the cavity distribution functions for the hard sphere interaction site fluids. If the site-site formalism is being used, then the only suitable... 

The evidence available suggests that the two approaches are about equally accurate, although the approach based on site-site correlation functions is more readily generalized to the treatment of multipolar interactions as well as to the effect of the attractive forces upon the structure and free energy at moderate and low density. In addition to the efforts made at extending the WCA theory to interaction site fluids, the Barker-Henderson theory has also been extended to these systems by Lombardero, Abascal, Lago and their co-workers. ... 

There has been a trend towards rather more attention being given to theoretical calculations of the structure of interaction site fluids than to their thermodynamic properties, although in recent years this trend has been somewhat reversed. In this section, we describe the efforts that have been... 

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