An exponential multi-reference wavefunction ansatz:: connectivity analysis and application to N2

A connectivity analysis for the exponential multi-reference wavefunction ansatz (MRexpT) (J Chem Phys 123:84102, 2005) is carried out. Assuming a complete model space and separating interactions carrying active labels the cluster operator carrying no active labels is fully connected. The valence (active) part of the MRexpT cluster operator, however remains disconnected. Consequently, the MRexpT correlation energy scales linearly with the number of non-active electrons as single reference coupled cluster does while MRexpT additionally can treat multi reference cases. Therefore, MRexpT should be well suited to be applied to a large number of molecular applications. Its applicability to periodic systems with multi-reference unit cells however seems to be limited. An application to the triple bond breaking of the N-2 molecule is presented.