A model spin-vibronic Hamiltonian for twofold degenerate electron systems: A variational ab initio study of X 2E CH3O•

A sophisticated spin-vibronic model was developed to study electronic and nuclear dynamics in twofold degenerate electron systems. Eigenenergies and eigenfunctions of a model Hamiltonian are calculated in a basis set of products of electronic, electron spin, and vibrational functions. The X̃ 2E ground electronic state of the CH3O pyramidal (C3v) system has been studied with the simultaneous treatment of spin-orbit coupling, all linear and quadratic Jahn-Teller interactions including multimode couplings, and anharmonic effects up to the sixth order for the CH–stretching. The group-theoretical analysis of the spin-vibronic Hamiltonian and its eigenfunctions was performed in terms of irreducible representations (E3/2 and E1/2) of the double C3v symmetry group. Vibronic and anharmonic model parameters of X̃ 2E CH3O were calculated with numerical differentiation using ab initio energies of the CH3O geometries distorted on normal coordinates. The equation-of-motion coupled cluster method with augmented core-valence basis sets of triple-ζ quality was applied in these calculations. The value of the spin-orbit splitting in X̃ 2E CH3O was calculated using multiconfiguration quasidegenerate second-order perturbation theory with a complete active space reference wave function followed by a perturbative calculation of eigenvalues of the full Breit-Pauli spin-orbit operator.