Improving perturbation theory for open-shell molecules via self-consistency

TL;DR

This paper introduces an extended self-consistent OBMP2 method for open-shell molecules that improves upon traditional MP2 by allowing orbital relaxation, leading to more accurate predictions in challenging cases like bond breaking.

Contribution

The paper develops a self-consistent OBMP2 approach for open-shell systems, combining canonical transformation and cumulant approximation to enhance accuracy over non-iterative MP2.

Findings

OBMP2 provides smooth energy transitions during bond breaking.

OBMP2 accurately predicts isotropic hyperfine coupling constants.

Self-consistency improves orbital relaxation in open-shell molecules.

Abstract

We present an extension of our one-body M{\o}ller-Plesset second-order perturbation (OBMP2) method for open-shell systems. We derived the OBMP2 Hamiltonian through the canonical transformation followed by the cumulant approximation to reduce many-body operators into one-body ones. The resulting Hamiltonian consists of an uncorrelated Fock (unperturbed Hamiltonian) and a one-body correlation potential (perturbed Hamiltonian) composed of only double excitations. Molecular orbitals and associated energy levels are then relaxed via self-consistency, similar to Hartree-Fock, in the presence of the correlation at the MP2 level. We demonstrate the OBMP2 performance by considering two examples well known for requiring orbital optimization: bond breaking and isotropic hyperfine coupling constants. In contrast to non-iterative MP2, we show that OBMP2 can yield a smooth transition through the unrestriction point and accurately predict isotropic hyperfine coupling constants.