Analytical determination of non-born—oppenheimer matrix elements between different molecular electronic states

Matrix elements for the nonadiabatic correction between different adiabatic Born—Oppenheimer (ABO) states are discussed in terms of a generating function (nonadiabatic coupling function, NAF) which was introduced earlier. Recursion relations are derived for the case where different vibronic states are coupled via a single non-totally symmetric mode. The formalism is applied to the estimation of non-Born—Oppenheimer coupling between vibronic states of pyrazine.