Benchmark Quasi-Variational Coupled Cluster Calculations of Multiple Bond Breaking

We present further evidence that closed-shell single-reference coupled cluster theory can be used as a reliable predictive tool even when multiple chemical bonds are broken, provided that a near-variational, rather than a projective, ansatz is used. Building on the Optimized-orbital Quasi-Variational Coupled Cluster Doubles (OQVCCD) method by adding the standard perturbative treatment of triple excitations, the OQVCCD(T) method provides outstanding accuracy for the dissociation of multiply bonded molecules and other problems involving strong nondynamic correlation of the electrons. We find that in the case of singly bonded molecules, OQVCCD and OQVCCD(T) perform similarly to the equivalent Brueckner Coupled Cluster Doubles approaches, BCCD and BCCD(T). However, when multiple bonds are broken, such as in acetylene and dicarbon, OQVCCD(T) is capable of predicting both qualitatively and quantitatively accurate potential energy curves, unlike the standard methods based on traditional coupled cluster theory, and for approximately the same computational cost.