The group product function ansatz of McWeeny serves as a prototype model for uncorrelated groups of orbitals. It assumes partitioning of orbitals into disjoint sets. A given group is embedded in the field of the other groups and the electron interaction among them is described in the mean field manner. Many widely used models of multireference wavefunctions conform to the group-product-function form, the examples being: strongly orthogonal geminal models (APSG and GVB-PP), complete active space (CAS), some of the MCSCF and DMRG models. The usefulness of all of them is hindered by the fact that they miss inter-orbital-group electron correlation.<br/>Considering coupling of density fluctuations of strongly orthogonal geminals an employing extended random phase approximation (ERPA) to describe linear response properties have led to formulation of inter-geminal correlation correction [1]. It has tuned out that the correlation energy correction follows also from the adiabatic connection (AC) formalism for multireference wavefunctions [2]. More recently we have shown that the AC-ERPA method yields excellent results for ground and excited states of molecules described with the CASSCF wavefunction [3,4]. The AC-ERPA method competes or surpasses in accuracy the perturbation (PT2 or NEVPT2) corrections. Unlike the latter methods the AC-ERPA does not require construction of higher than two-particle reduced density matrices so it avoids the computational bottleneck of the perturbation methods.

Correlation energy for embedded orbital groups

2019 Spring National Meeting

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