Renormalized Internally-Contracted Multireference Coupled Cluster with Perturbative Triples

TL;DR

This paper introduces the ric-MRCC method, combining multireference coupled cluster with renormalization techniques, achieving high accuracy in molecular energy calculations with improved stability and efficiency.

Contribution

It develops the renormalized ic-MRCC (ric-MRCC) method by adapting flow equation renormalization to nonunitary transformations and introduces approximate triples for enhanced accuracy.

Findings

ric-MRCCSD[T] matches CCSD(T) accuracy for spectroscopic constants.

The method accurately models potential energy curves of various molecules.

It offers a stable, efficient approach for multireference quantum chemistry calculations.

Abstract

In this work, we combine the many-body formulation of the internally contracted multireference coupled cluster (ic-MRCC) method with Evangelista's multireference formulation of the driven similarity renormalization group (DSRG). The DSRG method can be viewed as a unitary multireference coupled cluster theory, which renormalizes the amplitudes based on a flow equation approach to eliminate numerical instabilities. We extend this approach by demonstrating that the unitary flow equation approach can be adapted for nonunitary transformations, rationalizing the renormalization of ic-MRCC amplitudes. We denote the new approach, the renormalized ic-MRCC (ric-MRCC) method. To achieve high accuracy with a reasonable computational cost, we introduce a new approximation to the Baker-Campbell-Hausdorff expansion. We fully consider the linear commutator while approximating the quadratic commutator, for which we neglect specific contractions involving amplitudes with active indices. Moreover, we introduce approximate perturbative triples to obtain the ric-MRCCSD[T] method. We demonstrate the accuracy of our approaches in comparison to advanced multireference methods for the potential energy curves of H8, F2, H2O, N2, and Cr2. Additionally, we show that ric-MRCCSD and ric-MRCSSD[T] match the accuracy of CCSD(T) for evaluating spectroscopic constants and of full configuration interaction energies for a set of small molecules.