Assessing the accuracy of tailored coupled cluster methods corrected by electronic wave functions of polynomial cost

TL;DR

This study evaluates the performance of tailored coupled cluster methods corrected by polynomial-cost electronic wave functions, demonstrating their effectiveness over traditional methods in describing complex molecular systems.

Contribution

It introduces and assesses the accuracy of DMRG- and pCCD-tailored coupled cluster methods for small- and medium-sized molecules, outperforming conventional CCSD in many cases.

Findings

DMRG- and pCCD-tailored methods yield similar errors to multireference data.

These methods generally outperform CCSD in spectroscopic and potential energy surface predictions.

fpCC methods are reliable for certain bond dissociations but fail in others.

Abstract

Tailored coupled cluster theory represents a computationally inexpensive way to describe static and dynamical electron correlation effects. In this work, we scrutinize the performance of various tailored coupled cluster methods externally corrected by electronic wave functions of polynomial cost. Specifically, we focus on frozen-pair coupled-cluster (fpCC) methods, which are externally corrected by pair-coupled cluster doubles (pCCD), and coupled cluster theory tailored by matrix product state wave functions optimized by the density matrix renormalization group (DMRG) algorithm. As test system, we selected a set of various small- and medium-sized molecules containing diatomics (N$_2$, F$_2$, C$_2$, CN$^+$, BN, BO$^+$, and Cr$_2$) and molecules (ammonia, ethylene, cyclobutadiene, benzene) for which conventional single-reference coupled cluster singles and doubles (CCSD) is not able to produce accurate results for spectroscopic constants, potential energy surfaces, and barrier heights. Most importantly, DMRG-tailored and pCCD-tailored approaches yield similar errors in spectroscopic constants and potential energy surfaces compared to multireference and/or experimental reference data and generally outrank the conventional single-reference CCSD approach. Although fpCC methods provide a reliable description for the dissociation pathway of molecules featuring single and quadruple bonds, they fail in the description of triple or hextuple bond-breaking processes or avoided crossing regions.