Spin-forbidden chemistry within the Breit-Pauli approximation

Abstract

Abstract Recent advances in the computational techniques available to characterize spin-forbidden processes with the Breit-Pauli approximation are reviewed and the use of these techniques is illustrated. These computational advances include: (i) The use of symbolic matrix element techniques to evaluate matrix elements of the full microscopic spin-orbit Hamiltonian (both the spin-orbit and spin-other-orbit terms) and the dipolar spin-spin Hamiltonian. This approach permits the Breit-Pauli interaction to be characterized in terms of large configurations state functions (CSF) spaces ≥ 106 terms. (ii) The relativistic wavefunctions are determined, directly in the CSF basis, using quasi-degenerate perturbation theory. This approach avoids the computational bottleneck which occurs if the perturbed wavefunction is determined in the eigenstate basis of H0 the non-relativistic Born-Oppenheimer Hamiltonian. (iii) The use of a Lagrange-Newton, analytic gradient-Mcscf/CI wavefunction based, algorithm for determining ...