A hybrid stochastic configuration interaction-coupled cluster approach for multireference systems

TL;DR

This paper introduces a hybrid stochastic approach combining configuration interaction and coupled cluster methods to improve the accuracy and efficiency of multireference electronic structure calculations.

Contribution

It develops a new class of methods that integrate configuration interaction ideas into the mrCCMC framework, enhancing stability and reducing computational cost.

Findings

New methods offer better stability and cost-accuracy balance.

Increased relaxation of the reference space improves solution exploration.

Framework enhances understanding of mrCCMC solutions.

Abstract

The development of multireference coupled cluster (MRCC) techniques has remained an open area of study in electronic structure theory for decades due to the inherent complexity of expressing a multi-configurational wavefunction in the fundamentally single-reference coupled cluster framework. The recently developed multireference coupled cluster Monte Carlo (mrCCMC) technique uses the formal simplicity of the Monte Carlo approach to Hilbert space quantum chemistry to avoid some of the complexities of conventional MRCC, but there is room for improvement in terms of accuracy and, particularly, computational cost. In this paper we explore the potential of incorporating ideas from conventional MRCC - namely the treatment of the strongly correlated space in a configuration interaction formalism - to the mrCCMC framework, leading to a series of methods with increasing relaxation of the reference space in the presence of external amplitudes. These techniques offer new balances of stability and cost against accuracy, as well as a means to better explore and better understand the structure of solutions to the mrCCMC equations.