Exchange-Dispersion Energy: A Formulation in Terms of Monomer Properties and Coupled Cluster Treatment of Intramonomer Correlation.

A new formulation of the second-order exchange-dispersion energy of symmetry-adapted perturbation theory is presented. The proposed approach allows to study - for the first time for many-electron monomers - the effect of intramonomer electron correlation on this energy correction. In the new formalism the exchange-dispersion energy is expressed in terms of properties of interacting molecules, such as dynamic density-matrix susceptibilities and one-electron reduced density matrices. The resulting formula has been used to obtain the main (noncumulant) part of the exchange-dispersion energy for monomers described by coupled cluster theory limited to single and double excitations. A density fitting approach has been applied in order to reduce the computational effort for obtaining coupled cluster density-matrix susceptibilities. The new coupled cluster exchange-dispersion energy has been compared with other available values of this interaction energy component, obtained with monomers treated on the level of Hartree-Fock or density-functional theories.