Externally Corrected CCSD with Renormalized Perturbative Triples (R-ecCCSD(T)) and the Density Matrix Renormalization Group and Selected Configuration Interaction External Sources

We
investigate the renormalized perturbative triples correction
together with the externally corrected coupled-cluster singles and
doubles (ecCCSD) method. We use the density matrix renormalization
group (DMRG) and heat-bath CI (HCI) as external sources for the ecCCSD
equations. The accuracy is assessed for the potential energy surfaces
of H2O, N2, and F2. We find that
the triples correction significantly improves upon ecCCSD, and we
do not see any instability of the renormalized triples with respect
to dissociation. We explore how to balance the cost of computing the
external source amplitudes against the accuracy of the subsequent
CC calculation. In this context, we find that very approximate wave
functions (and their large amplitudes) serve as an efficient and accurate
external source. Finally, we characterize the domain of correlation
treatable using the ecCCSD and renormalized triples combination studied
in this work via a well-known wave function diagnostic.