Coupled-cluster approach to spontaneous symmetry breaking in molecules:: The linear N3 radical

Symmetry-breaking phenomena that often affect calculation of potentials for the linear ABA species along the antisymmetric-stretching coordinate are examined for the case of the ground state of the N-3 radical using various coupled-cluster methods and molecular orbitals. The difficulties that are encountered when employing UHF-type approaches are found to be particularly striking. It is shown that the inconsistencies that arise when relying on a numerical differentiation can be overcome by exploiting the computed potentials in the Schrodinger equation for the nuclear motion. This approach yields reliable values for the vibrational levels even in the presence of singularities that plague the UHF-based potentials. The variety of the symmetry-breaking phenomena that occur for these systems, including the notoriously intricate BNB radical, are briefly discussed. (c) 2008 Wiley Periodicals, Inc.