Multireference determinant CI calculations and CASSCF calculations on the CH3-NO2 decomposition pathway of nitromethane

The multideterminant character of CH3NO2. at its ground-state equilibrium geometry, involving the ground SCF state plus a doubly excited (π*-like) [2] configuration out-of-plane with respect to the plane of the NO2 group, has been studied by MRD-CI (multireference determinant configuration interaction) and CASSCF (complete active space SCF) methods to elucidate the configurations required for an accurate description of the system along the decomposition pathway of the C-N bond. In the MRD-CI work, the ground and three lowest electronically excited states were investigated at a number of values of r(C-N); at each point about 850,000 configurations were generated by single and double excitations, of which ˜4500 were included explicitly in the larger MRD-CI wave function by selection criteria. Perturbation extrapolation procedures then provided the correction for the omitted configurations, and this was augmented/supplemented by a Davidson-type correction for the higher than double excitations. The CASSCF investigation included seven active orbitals. Both the MRD-CI results and the CASSCF results verified our earlier findings of the two-determinant character of CH3NO2 even in its ground electronic state at equilibrium geometry. Further, both these results indicate that a number of additional determinants become important along the C-N decomposition pathway.