Degeneracy breaking in the Hilbert-space coupled cluster method

The Hilbert space multireference coupled cluster method with single and double excitations (HS–MRCCSD) is applied to excited states of the N2 molecule. Particular attention is given to the breaking of degeneracy, where sublevels which should be degenerate by symmetry come out separated by as much as 0.3 eV. This feature is explained by the fact that a truncated coupled cluster expansion starting from an open-shell determinant does not preserve spin or spatial symmetry.