Second-order exchange-dispersion energy based on multireference description of monomers

TL;DR

This paper introduces a new method to calculate second-order exchange-dispersion energies for weakly interacting monomers with multireference wave functions, using ERPA and SAPT, applicable to systems with static correlation or excited states.

Contribution

The paper develops a formalism based on ERPA within SAPT for multireference monomers, extending the calculation of exchange-dispersion energies to more complex systems.

Findings

Accurate exchange-dispersion energies for ground-state multireference dimers.

Large errors in excited-state hydrogen-helium interactions due to approximation limitations.

Abstract

We present a method for calculation of the second-order exchange-dispersion energy in the framework of the symmetry-adapted perturbation theory (SAPT) for weakly interacting monomers described with multiconfigurational wave functions. The proposed formalism is based on response properties obtained from extended random phase approximation (ERPA) equations and assumes the single-exchange (S2) approximation. The approach is applicable to closed shell systems where static correlation cannot be neglected or to systems in nondegenerate excited states. We examine the new method in combination with either generalized valence bond perfect pairing (GVB) or complete active space self consistent field (CASSCF) description of the interacting monomers. For model multireference dimers in ground-states (H2-H2, Be-Be, He-H2) exchange-dispersion energies are reproduced accurately. For the interaction between the excited hydrogen molecule and the helium atom we found unacceptably large errors which is attributed to the neglect of diagonal double excitations in the employed approximation to the linear response function.