Properties of nonadiabatic couplings and the generalized Born-Oppenheimer approximation

We present a new analysis of the nonadiabatic coupling terms in the coupled equations for nuclear motion wave functions when the Born-Oppenheimer (BO) representation is used for the electronic wave function. The new analysis leads to a criterion for truncating the series and neglecting terms in the coupled equations of motion. We show that in general the nonremovable part of the coupling is of the same magnitude as the removable part, except near intersections of the adiabatic states. (C) 2002 Elsevier Science B.V. All rights reserved.