On the use of the Breit–Pauli approximation for evaluating line strengths for spin-forbidden transitions: Application to NF

Abstract

An ab initio treatment of the Breit–Pauli spin-orbit interaction is presented for use with large (state-averaged) MCSCF/CI wave functions and Cartesian Gaussian atomic orbitals. The first order perturbation theory equation which results from treating the Breit–Pauli interaction (Hso) as a perturbation to the nonrelativistic Born–Oppenheimer Hamiltonian (H0) is formulated in the configuration state function basis thereby permitting the coupling of all roots of H0 into the perturbed wave function. The basic atomic orbital integrals of Hso over Cartesian Gaussian functions are evaluated using a Rys quadrature approach. The method presented here is used to consider the spin forbidden transitions b 1Σ+→X 3Σ− and a 1Δ→X 3Σ− in NF. A comparison with a frequently used approach based on the few lowest eigenfunctions of H0 is presented.