Towards the ab initio determination of strictly diabatic states, study for (NaRb)+

A generalization of the effective metric approach is proposed and applied both for the calculation of radial couplings and for the determination of diabatic states along a single coordinate using the formalism of variational effective Hamiltonian theory. The application to the ionic molecule NaRb+ shows that strictly diabatic states are obtained even for very short distances where a huge number of crossings are observed. Polarization and electronic delocalization effects are estimated. A comparison with various diabatization methods is performed. The proposed method brings a significant improvement with respect to the existing ones. (C) 1999 American Institute of Physics. [S0021-9606(99)30913-2].