Quasi-chemical theories of associated liquids

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Abstract
It is shown how traditional development of theories of fluids based upon the concept of physical clustering can be adapted to an alternative local clustering definition. The alternative clustering definition can preserve a detailed valence description of the interactions between a solution species and its near-neighbours, i.e., cooperativity and saturation of coordination for strong association. These clusters remain finite even for condensed phases. The simplest theory to which these developments lead is analogous to quasi-chemical theories of cooperative phenomena. The present quasi-chemical theories require additional consideration of packing issues because they do not impose lattice discretizations on the continuous problem. These quasi-chemical theories do not require pair decomposable interaction potential energy models. Since calculations may be required only for moderately sized clusters, it is suggested that these quasi-chemical theories could be implemented with computational tools of current molecular electronic structure theory. In this way an intermediate step of approximate force field generation can be avoided.