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

The short answer to the question in title is “no”. We identify classes of truncated configuration interaction (CI) wave functions for which externally corrected coupled-cluster (ec-CC) approach using three-body (T3) and four-body (T4) components cluster operator extracted from CI does not improve results underlying calculations. Implications our analysis, illustrated by numerical examples, ec-CC computations selected methods are discussed. also introduce a novel T3 T4 amplitudes obtained with scheme abbreviated as CIPSI, correcting resulting energies missing correlations captured CIPSI help moment expansions similar those employed completely renormalized CC methods.