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 f avorably compared to available experimental and previous theoretical work. The lowest two ionization potentials are accurately determined using wave f unctions constructed as simple single and double substitutions from a self- consistent field reference configuration with scalar relativistic effects i ncluded through an averaged relativistic pseudopotential. A determination o f the third ionization potential to comparable accuracy requires inclusion of the spin-orbit operator and significant intermediate coupling with a res ulting configuration expansion length in excess of 1.9 million double-group adapted functions. The solution to this problem was achieved by applicatio n of a new parallel spin-orbit configuration interaction component to the C OLUMBUS Program System. A decomposition of the ionization potential calcula tion into parts either sensitive or largely insensitive to the spin-orbit o perator is favorably tested, leading to hybrid calculations of improved acc uracy. (C) 2000 American Institute of Physics. [S0021-9606(00)30303-8].