  We present a model for molecular materials made up of polar and polarizable
molecular units. A simple two state model is adopted for each molecular site
and only classical intermolecular interactions are accounted for, neglecting
any intermolecular overlap. The complex and interesting physics driven by
interactions among polar and polarizable molecules becomes fairly transparent
in the adopted model. Collective effects are recognized in the large variation
of the molecular polarity with supramolecular interactions, and cooperative
behavior shows up with the appearance, in attractive lattices, of discontinuous
charge crossovers. The mean-field approximation proves fairly accurate in the
description of the gs properties of MM, including static linear and non-linear
optical susceptibilities, apart from the region in the close proximity of the
discontinuous charge crossover. Sizeable deviations from the excitonic
description are recognized both in the excitation spectrum and in linear and
non-linear optical responses. New and interesting phenomena are recognized near
the discontinuous charge crossover for non-centrosymmetric clusters, where the
primary photoexcitation event corresponds to a multielectron transfer.
