Ionization potentials of alkali atoms: towards meV accuracy

Ionization potentials of orbitals in the three shells above the cation closed-shell structure (17 IPs per atom) are calculated for the alkali atoms Rb, Cs, Fr and element 119. The Dirac-Coulomb-Breit Hamiltonian is used as framework, and correlation is included by the multireference Fock-space coupled cluster method with single and double excitations. The basis includes 36s32p24d22f10g7h6i Gaussian-type orbitals. Comparison with the 45 experimentally known IPs shows excellent agreement, with an average absolute error of 11 cm−1 (1.4 meV) or 0.10% and maximum error of 81 cm−1 (10 meV) or 0.25%. The largest errors occur in the ground state IPs. Correcting for the Lamb shifts improves the s orbital binding energies, reducing the maximum error to 60 cm−1 (7.4 meV) or 0.19%. The remaining error is attributed to the truncation of the coupled cluster expansion and, to lesser extent, to small deficiencies in the basis.