Abstract We study the nonradiative processes in diatomic molecules produced by a potential surface crossing in terms of a generalized optical potential containing an absorptive and a resonant part. The theory is applicable to inelastic atomic scattering, predissociation, accidental predissociation and collisionally induced dissociation. The coupling term between the electronic surfaces (in the diabatic representation) is evaluated semiclassically in terms of the “inelastic action” s . While the Landau–Zener formula is obtained from a linearized function of s , a more realistic form is proposed as a rational fraction in s .