  We investigate the renormalized perturbative triples correction together with
the externally corrected coupled-cluster singles and doubles (ecCCSD) method.
We take the density matrix renormalization group (DMRG) and heatbath 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 on 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 with respect to
the accuracy of the subsequent CC calculation. In this context, we find that
very approximate wavefunctions (and their large amplitudes) serve as an
efficient and accurate external source. Finally, we characterize the domain of
correlation treatable using the externally corrected method and renormalized
triples combination studied in this work via a well-known wavefunction
diagnostic.
