State-Specific Multireference Coupled Cluster-Based Methods for Potential Energy Surfaces and Their Approximate Variants

Abstract

Traditional multi-reference (MR) coupled-cluster approaches based on the effective hamiltonian formalism are often plagued with intruder states, and are generally unsuitable for potential energy surface (PES) studies. We present here a concise account of our recently developed state-specific multi-reference coupled-cluster (SS-MRCC) theory and its certain approximate variants, which are size-extensive and size-consistent and are designed to obviate intruders in a natural manner. The approximations include the state-specific perturbative theories (SS-MRPT) and the coupled electron-pair approximation (SS-MRCEPA) versions, which retain the essential physics without losing much of the accuracy of the full-blown SS-MRCC. All these methods are based on a complete active space (CAS), and the target energy is obtained by diagonalizing an effective operator in this space. The methods are free of intruders whenever the target state energy is well separated from the virtual functions and work smoothly over wide range of geometries of various molecular states having pronounced MR