Semiempirical spin–orbit coupling calculations. I. Theory and method. Benzophenone as a test case

The molecular spin–orbit coupling operator is brought into a simplified form through a convenient choice of origin for the orbital angular momentum operator. The eigenvalue problem of the Hamiltonian that includes the spin–orbit (SOC ) operator as a perturbation is solved by means of a linear variational procedure in the basis of the spin-pure molecular eigenstates. Test calculations on benzophenone are presented and the results are compared to experiment. We discuss the minimal size of the spin-pure variational basis needed to achieve stable results as well as the amount of single-excitation configurational mixing needed to describe the spin-pure molecular eigenstates.