Configuration interaction in rare-earth ions. I. Three particle correlation in Pr²⁺

Coulomb matrix elements are obtained for configuration interaction processes in Pr 2+ and Pr 3+ , using Hartree-Fock-Slater wave functions. Second order perturbation theory is used to calculate the six parameters which characterize the effective three-particle operator for Pr 2+ . The results are compared with parameters obtained in a recent analysis of experimental data. 4f to nh processes are shown to be negligible; the contributions due to 4f to nf processes are calculated to within 20% of their true values; higher order perturbation terms are shown to be necessary for an adequate determination of the contributions due to the 4f to or from np processes.