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

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 (T3) and four-body (T4) 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 T3 and T4 amplitudes obtained with the selected CI scheme abbreviated as CIPSI, correcting the resulting energies for the missing T3 correlations not captured by CIPSI with the help of moment expansions similar to those employed in the completely renormalized CC methods.