Symmetry-adapted perturbation theory based on multiconfigurational wave function description of monomers

TL;DR

This paper introduces a multiconfigurational symmetry-adapted perturbation theory (SAPT) for accurately modeling noncovalent interactions involving strongly correlated or excited monomers, extending SAPT to complex electronic states.

Contribution

The paper develops SAPT(MC), a novel formulation that incorporates multiconfigurational wave functions and response properties, enabling accurate interaction energy calculations for complex systems.

Findings

SAPT(MC) reproduces benchmark results for multireference systems.

Negative transition contributions are essential for accurate energy predictions.

The method effectively handles excited-state and strongly correlated monomers.

Abstract

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.