A multi-reference coupled-cluster study on the potential energy surface of N2 including ground and excited states: spin projections and wavefunction overlaps

This article reports on the calulation of 12 low lying states of the nitrogen molecule along its dissociation using the multi-reference exponential wavefunction ansatz (Hanrath in J Chem Phys 123:84102, 2005), the single-reference formalism multi-reference coupled cluster (Oliphant and Adamowicz in J Chem Phys 94:1229, 1991), and MRCI methods. Spin projection errors and state overlap errors are calculated and allow an analysis of the wavefunction with respect to properties different from correlation energies. Both criteria are very sensitive to errors in the wavefunction. Due to its lack of Fermi vacuum invariance the errors are more significant for the single-reference formalism based approach.