PNNL

Abstract

Systematic sequences of basis sets are used to calculate the spin-orbit splittings of the halogen atoms F, Cl, and Br in the framework of first order perturbation theory with the Breit-Pauli operator and internally contracted configuration interaction wave functions. The effects of both higher angular momentum functions and the presence of tight functions are studied. By systematically converging the 1-particle basis set, an unambiguous evaluation of the effects of correlating different numbers of electrons in the CI treatment is carried out. Correlation of the 2p-electrons in chlorine increases the spin-orbit splitting by ~80 cm-1, while in bromine we observe incremental increases of 130 cm?1, 145 cm-1, and 93 cm-1, when adding the 3d-, 3p-, and 2p-electrons to the set of explicitly correlated electrons, respectively. For fluorine and chlorine the final basis set limit, all-electrons correlated results match the experimentally observed spin-orbit splittings to within ~5 cm-1, while for bromine the Breit-Pauli operator underestimates the splitting by about 100 cm-1. More extensive treatment of electron correlation results in only a slight lowering of the spin-orbit matrix elements. Thus, the discrepancy for bromine is proposed to arise from the nonrelativistic character of the underlying wave function.