Quasi-variational coupled cluster theory.

Statistical analysis of activation and reaction energies with quasi-variational coupled-cluster theory

Application of the quasi-variational coupled cluster method to the nonlinear optical properties of model hydrogen systems.

We present a pilot application of the recently proposed quasi-variational coupled cluster method to the energies, polarizabilities, and second hyperpolarizabilities of model hydrogen chains. Relative to other single-reference methods of equivalent computational complexity, we demonstrate this method to be highly robust and especially useful when traditional coupled cluster theory fails to perfo...

Multireference Approximate Variational Coupled Cluster Theories

Multireference Configuration Interaction (MRCI) is widely and successfully used for the accurate computation of molecular energies and properties when chemical bonds are broken, and in other cases of quasi-degeneracy. However, it suffers from a lack of extensivity, which introduces significant errors for large molecules. Currently feasible Multireference Coupled Cluster (MRCC) approaches, which...

Breaking multiple covalent bonds with Hartree-Fock-based quantum chemistry: Quasi-Variational Coupled Cluster theory with perturbative treatment of triple excitations.

We enhance the recently proposed Optimized-orbital Quasi-Variational Coupled Cluster Doubles (OQVCCD) method for the calculation of ground-state molecular electronic structure by augmenting it with the standard perturbative (T) correction for the effects of connected triple excitations. We demonstrate the OQVCCD(T) ansatz to be outstandingly robust and accurate in the description of the breakin...

Rigorously extensive orbital-invariant renormalized perturbative triples corrections from quasi-variational coupled cluster theory.

We show that, by making use of the linked tensor objects inherent to the approach, Orbital-optimised Quasi-Variational Coupled Cluster Theory (OQVCCD) leads naturally to a computationally-trivial, rigorously extensive, and orbital-invariant renormalization of the standard (T) correction for the perturbative inclusion of the effects of connected triple excitations. The resulting prototype method...