A coupled cluster and full configuration interaction study of CN and CN−

Full configuration interaction (FCI) and coupled cluster (CC) calculations are carried out for the CN radical and CN− using the cc-pVDZ and an augmented cc-pVDZ basis set. In addition, CC calculations including up to quadruple excitations are carried out using the cc-pVTZ basis. At the FCI level, the equilibrium distance is 1.1969 Å, the harmonic frequency is 2020.1 cm−1, the electronic contribution to the atomization energy is 667 kJ/mol and the vertical electron affinity is 0.12962 Eh. The contributions from quadruple and quintuple excitations to the harmonic frequency are found to be 20 and 5 cm−1, respectively. The quadruple excitations give a contribution of 4 kJ/mol to the atomization energy and 0.00013 Eh to the vertical electron affinity. None of the calculations indicate that the convergence of the CC hierarchy is slower for open-shell than for closed-shell systems.