  We present a formulation of the multiconfigurational (MC) wave function
symmetry-adapted perturbation theory (SAPT). The method is applicable to
noncovalent interactions between monomers which require a multiconfigurational
description, in particular when the interacting system is strongly correlated
or in an electronically excited state. SAPT(MC) is based on one- and
two-particle reduced density matrices of the monomers and assumes the
single-exchange approximation for the exchange energy contributions.
Second-order terms are expressed through response properties from extended
random phase approximation (ERPA) equations. SAPT(MC) is applied either with
generalized valence bond perfect pairing (GVB) or with complete active space
self consistent field (CASSCF) treatment of the monomers. We discuss two model
multireference systems: the H2-H2 dimer in out-of-equilibrium geometries and
interaction between the argon atom and excited state of ethylene. In both cases
SAPT(MC) closely reproduces benchmark results. Using the C2H4-Ar complex as an
example, we examine second-order terms arising from negative transitions in the
linear response function of an excited monomer. We demonstrate that the
negative-transition terms must be accounted for to ensure qualitative
prediction of induction and dispersion energies and develop a procedure
allowing for their computation. Factors limiting the accuracy of SAPT(MC) are
discussed in comparison with other second-order SAPT schemes on a data set of
small single-reference dimers.
