Ab Initio Determination of Americium Ionization Potentials

The first three ionization potentials of americium are calculated using ab initio spin-orbit configuration interaction techniques. These results are favorably compared with experimental and theoretical work. The lowest two ionization potentials are accurately determined using wavefunctions constructed as simple single and double substitutions from a self-consistent field reference configuration with scalar relativistic effects included through an averaged relativistic pseudopotential. A determination of the third ionization potential to comparable accuracy requires inclusion of the spin-orbit operator and significant intermediate coupling, with a resulting configuration expansion length in excess of 1.9 million double-group adapted functions. This problem was solved by application of a new parallel spin-orbit configuration interaction component to the COLUMBUS program system. A decomposition of the ionization potential calculation into parts either sensitive or largely insensitive to the spin-orbit operator was favorably tested, leading to hybrid calculations of improved accuracy.