paper

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

arXiv:2102.10143 · doi:10.1021/acs.jctc.1c00181

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.

70 pages, 4 tables, 2 charts, and 1 figure. Supporting Information as an ancillary file. This article has been accepted for publication in the Journal of Chemical Theory and Computation