PNNL

Abstract

A new form of the asymmetric energy functional for the multireference coupled cluster (MRCC) theories is discussed from the point of view of an energy expansion in quasidegenerate situation. The resulting expansion for the exact electronic energy can be used to define the non-iterative corrections to approximate MRCC approaches. In particular, we show that in the proposed framework the essential part of dynamic correlation can be encapsulated in the so-called correlation Hamiltonian, which in analogy to the effective Hamiltonian, is defined in the model space (M0). When the trail wavefunction (wavefunctions) is determined without invoking a specific form of the MRCC sufficiency conditions, the ensuing correction can be universally applied to any type of the approximate MRCC method employing the same excitation manifold. We also discuss several strategies of normalizing trial wavefunctions, which significantly simplify the algebraic structure of the correction. The analogies with other MRCC triples corrections to MRCC theories with singles and doubles are outlined. Possible parallel algorithms for the non-iterative corrections are alsodiscussed.