Molecular closure approximations tail potentials

Taken as a whole, the ideas discussed led Yethiraj and Schweizer to propose the following reference molecular closure approximations for site interaction potentials consisting of a hard core plus tail" ... 

The development of molecular closure approximations was guided by three considerations (i) the indirect two-molecule correlation processes were explicitly included in the closure relation, (ii) contributions of the hard core and attractive tail parts of the potential were treated via separate approximations, and (iii) the closures recover exactly the two-molecule correlation processes in the high temperature weak coupling limit. The simplest approximation which incorporates the above three points is... 

This same idea is used in the development of molecular closure approximations by Sch weizer and Yethiraj [90]. The hard core reference system is treated with the PY closure. The closure for the attractive tail part of the potential is formulated to exactly describe the weak coupling limit. The simplest molecular closure that correctly treats the longer ranged potentials (i.e., gives Rory-Huggins scaling) is given by... 

The strategy for explicitly formulating the molecular closures was guided by three considerations." (1) Use of the commonly employed reference approach. The successful site-site PY closure is retained to describe the repulsive force reference fluid but a molecular closure scheme is adopted to describe the attractive, slowly varying forces. (2) The approximation scheme is required to provide an exact description of the structural consequences of the tail potentials at the two-molecule level in the weak coupling limit [/3umm-W 1]- (3) Use of an appropriate site-site approximation for the direct attractive interaction contribution motivated by experience in simple fluids. ... 

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