Dissociating N-2: a multi-reference coupled cluster study on the potential energy surfaces of ground and excited states

This article reports on the calculation of 12 low lying states of the nitrogen molecule along its dissociation using the multi-reference exponential wavefunction ansatz [J. Chem. Phys. 123, 84102 ( 2005)], the single-reference formalism multi-reference coupled cluster [ J. Chem. Phys. 94, 1229 ( 1991)], and MRCI methods. Energies relative to full CI are given. The results show the multi-reference coupled cluster approaches generally applicable to very demanding problems at high accuracy. In comparison to MRCI both coupled cluster type approaches do not reproduce spatial symmetry exactly. Nevertheless, this error in the degeneracy is much less than the overall error of the MRCI calculations.