An INDO/S‐CI treatment including spin‐orbit interaction based on Rumer spin functions. Application to the hydrated cerium ion

We examine the inclusion of spin‐orbit effects within the Rumer configuration interaction technique and discuss its implementation in the spectroscopic version of the intermediate neglect of differential overlap model (INDO/S‐CI). An efficient strategy for calculating excitation energies, transition moments, and Mulliken populations for Rumer‐adapted functions is described. As an example, results are presented for the ground and excited states of the hydrated trivalent cerium ion [Ce(H₂O)₉]³⁺, which confirm the low energy assignments to 4ƒ → 5 d transitions split by some 10000 cm⁻¹ through spin‐orbit coupling and ligand field interaction. Comparisons are made between this technique and one that we have used previously that utilizes configuration interaction over double‐group adapted linear combinations of determinants.