  The quasi-chemical organization of the potential distribution theorem --
molecular quasi-chemical theory (QCT) -- enables practical calculations and
also provides a conceptual framework for molecular hydration phenomena. QCT can
be viewed from multiple perspectives: (a) As a way to regularize an
ill-conditioned statistical thermodynamic problem; (b) As an introduction of
and emphasis on the neighborship characteristics of a solute of interest; (c)
Or as a way to include accurate electronic structure descriptions of
near-neighbor interactions in defensible statistical thermodynamics by clearly
defining neighborship clusters. The theory has been applied to solutes of a
wide range of chemical complexity, ranging from ions that interact with water
with both long-ranged and chemically intricate short-ranged interactions, to
solutes that interact with water solely through traditional van~der~Waals
interations, and including water itself. The solutes range in variety from
monoatomic ions to chemically heterogeneous macromolecules. A notable feature
of QCT is that in applying the theory to this range of solutes, the theory
itself provides guidance on the necessary approximations and simplifications
that can facilitate the calculations. In this Perspective, we develop these
ideas and document them with examples that reveal the insights that can be
extracted using the QCT formulation.
