QUASIDIABATIC STATES FOR INTRAMOLECULAR CHARGE-TRANSFER - APPLICATIONTO THE PROTONATION OF NH3

We have constructed ab initio adiabatic and quasidiabatic surfaces for the description of the protonation of NH3. For the diabatic states, w e applied a recently developed method which is based on the propagatio n along the reaction coordinate, starting from the dissociation region , of the maximized nonorthonormal overlap between diabatic states calc ulated at successive geometries. In agreement with earlier calculation s [Kaldor et al., J. Chem. Phys. 90, 6395 (1989)], it was found that t he adiabatic surfaces cannot explain this charge-transfer process. On the contrary, a single diabatic potential curve correlates smoothly th e ground state of NH4+ with the NH3+H+ limit.