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). The dispersion components of SAPT(MC) have been introduced in our previous works [HapkaM.J. Chem. Theory Comput.2019, 15, 1016−102730525591; HapkaM.J. Chem. Theory Comput.2019, 15, 6712–672331670950]. 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. 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.
Highlights
Quantum chemistry offers two complementary approaches to noncovalent interactions, the supermolecular approach and energy decomposition methods
A quantitative description of this system is challenging as it has to capture the balance between long-range dynamic correlation and increasing nondynamic correlation effects.[29,101]
In symmetry-adapted perturbation theory (SAPT)(CAS) calculations, each monomer is described with a complete active space (CAS)(2,5) wave function
Summary
Quantum chemistry offers two complementary approaches to noncovalent interactions, the supermolecular approach and energy decomposition methods. The former is conceptually simple and capable of providing the most accurate potential energy surfaces, e.g., for interpretation of experiments carried out in the cold- and ultracold regimes.[1−3] The latter, decomposition methods, allow insight into the nature of the interaction by partitioning the interaction energy into welldefined contributions. The main difficulty lies in the recovery of the remaining dynamic correlation both within and between the interacting molecules The latter effect, giving rise to the attractive dispersion interaction, poses a particular challenge due to its highly nonlocal and long-range nature. Many multireference methods restoring dynamic correlation effects have been developed, neither has yet managed to combine the accuracy and efficiency required for noncovalent interactions
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