Using a Lagrangian based approach, mathematical expressions were derived that analytically determine electric dipole moments, nuclear gradients, and nonadiabatic coupling coefficients for electronic states at the second-order generalized Van Vleck perturbation theory (GVVPT2) and the multireference configuration interaction (MRCI) levels of theory. The equations were expressed in a manner that ensures the slowest portions of the computer code are independent of the nuclear degrees of freedom that differentiate the energies and wavefunctions. The codes were implemented using a configuration-driven unitary group approach (UGA). Under this approach the efficient UGA method of evaluating configuration state function (CSF) matrix elements is recast within a macroconfiguration framework. The net result is a substantial increase in both the speed and flexibility of the conventional UGA procedure.;Finally GVVPT2 calculations were performed to study possible interactions between the singlet and triplet states that correspond to the first two dissociation channels of the (NO)2 → 2NO reaction. Potential energy curves were determined for each electronic state and the curves were used to identify possible intermediates, transition states, and regions where nonadiabatic coupling becomes energetically favorable.
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