Is Externally Corrected Coupled Cluster Always Better than the Underlying Truncated Configuration Interaction?

TL;DR

This paper investigates when externally corrected coupled-cluster methods improve upon truncated CI calculations, showing that in some cases they do not, and introduces a new ec-CC approach using selected CI amplitudes with corrections.

Contribution

The paper identifies classes of truncated CI wave functions where ec-CC does not outperform CI and proposes a novel ec-CC method using CIPSI amplitudes with moment-based corrections.

Findings

ec-CC does not always improve upon truncated CI results

Numerical examples illustrate limitations of ec-CC with certain CI classes

A new ec-CC approach using CIPSI amplitudes and moment corrections is introduced

Abstract

The short answer to the question in the title is 'no'. We identify classes of truncated configuration interaction (CI) wave functions for which the externally corrected coupled-cluster (ec-CC) approach using the three-body ($T_{3}$) and four-body ($T_{4}$) components of the cluster operator extracted from CI does not improve the results of the underlying CI calculations. Implications of our analysis, illustrated by numerical examples, for the ec-CC computations using truncated and selected CI methods are discussed. We also introduce a novel ec-CC approach using the $T_{3}$ and $T_{4}$ amplitudes obtained with the selected CI scheme abbreviated as CIPSI, correcting the resulting energies for the missing $T_{3}$ correlations not captured by CIPSI with the help of moment expansions similar to those employed in the completely renormalized CC methods.