Configuration interaction calculations of potential curves and annihilation rates for positronic complexes of alkali monoxides

Ab initio multireference single- and double-excitation configuration interaction calculations have been carried out to compute the potential curves and annihilation rates (ARs) of positronic molecular complexes of a series of alkali monoxides. The dissociation limit for the lowest states of these systems consists of the positive alkali ion ground state (M(+)) and the OPs (e(+)O(-)) complex formed by attaching the positron to O(-), even though the ground state of the corresponding neutral molecule always correlates with uncharged fragments (M+O). The positron affinity of the neutral oxide (2)Pi state is greater than that of (2)Sigma(+) in each case, so that the e(+)MO ground state always has (3,1)Pi symmetry, despite the fact that both KO and RbO have (2)Sigma(+) ground states. The bonding in the positronic systems is highly ionic at all internuclear distances and this causes their ARs to decrease gradually as the positive alkali ion approaches the OPs fragment.