We consider here two approaches which have been proposed in the literature to obtain diabatic states fromabinitiocalculations. First, by calculating explicitly the coupling vectorg=lang;psgr;2Verbar;nabla;psgr;1rang; which describes the nonadiabatic interaction between two adiabatic states psgr;1and psgr;2. Second, by some extrapolation process of the wave functions obtained at a particular reference point. The coupling vectorgis the sum of three contributions:g=gCI+gLCAO+gAO. The first two represent the change in character of the adiabatic states in the region of nonadiabatic coupling due to the variation of the CI and LCAO coefficients, whereasgAOresults from the translation of the atomic orbitals with the moving nuclear centers. Criteria have been given to recognize when it is possible to transform a set of CI wave functions into a pair of useful diabatic states. A particularly favorable situation is obtained when the interacting electronic states are doubly excited with respect to each other. Within the twohyphen;state approximation,gCIis strictly irrotational and never gives rise to problems. One can expect situations in which this is also true forgLCAO. However, that part of the coupling represented bygAOcan never be described as a rotation of two diabatic functions. Nevertheless, the latter contribution can frequently be neglected, at least when the nonadiabatic coupling is strong. The theory of the electron translation factors (ETFrsquo;s) provides further insight into the problem and confirms our conclusions.
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