Multireference coupled clusters
We develop a multireference extension of the coupled clusters (CC) method, which is based on the Brillouin-Wigner perturbation theory. The main objective is to develop a computationally tractable, reliable method for description of systems with strong static correlation, like diradicals, dissociating chemical bonds, some transition metal compounds, etc. As partial goals we have the inclusion of connected triple excitations, analytic gradient, and development of a tool for automatic implementation of multireference CC methods.
We are interested in theoretical aspects of multireference CC methods, particularly concerning the size-extensivity and intruder state problem. We are performing also application calculations of interesting chemical problems, for example the study of trimethylenemethane (TMM), or the Bergman reaction.
Example: The Ring-Opening of Methylenecyclopropane:
a Multireference Brillouin-Wigner Coupled Cluster Method
Methylenecyclopropane has a planar geometry. The ring opening has been studied by conrotatory and disrotatory paths (Fig.1). The trimethylenemethane (TMM) is a planar molecule with two nonbonding electrons. The two lowest excited states have a multireference character. It can be seen on the Fig.2 bellow.
Fig.1: Schema of the ring-opening of methylenecyclopropane
Fig.2: Results for the conrotation (MR BWCCSD vs. CCSD)