Coupled Cluster Treatment Of Intramonomer Correlation Effects In Intermolecular Interactions

Abstract

An adequate account of the effects of intramonomer correlation is indispensable to obtain an accurate representation of intermolecular potentials in symmetry-adapted perturbation theory (SAPT) calculations. These intramonomer correlation effects have initially been taken into account by employing Mø ller-Plesset perturbation theory, i.e., using the SAPT(MP) method, and more recently by applying density functional theory within the SAPT(DFT) approach. In this review a new approach, called SAPT(CC), is presented, in which the intramonomer correlation effects are treated by coupled cluster (CC) theory. Specifically, in the SAPT(CC) method each interaction energy component predicted by SAPT is expressed via monomer properties (density matrices, density susceptibilities and their generalizations) obtained from coupled cluster theory. In practice, the expectation-value approach to coupled cluster properties has been found most useful. The SAPT(CC) approach was implemented in practice at the SAPT(CCSD) level by including only singly and doubly excited parts of the cluster operator. At this level, the theory is exact for the interaction of two-electron monomers, i.e., takes into account (connected) triple and quadruple excitation contributions appearing in the supermolecular CC calculations of the interaction energy. The results obtained thus far using the SAPT(CCSD) approach are reviewed and compared with results of the corresponding SAPT(MP) and SAPT(DFT) treatments. The quality of the SAPT(CCSD) method is also examined by comparison with high-level supermolecular calculations performed using the CCSD(T), CCSDT(Q) and CCSDTQ methods.