GROUND AND EXCITED-STATES OF K-2 AND K-2+ BY THE OPEN-SHELL COUPLED-CLUSTER METHOD

The Fock space open-shell coupled cluster method is used to calculate the potential functions of the five lowest states of K2 at internuclear separation of 2 less-than-or-equal-to R less-than-or-equal-to 7 angstrom and the three lowest states of K2+ at 2 less-than-or-equal-to R less-than-or-equal-to 15 angstrom, with a 112-function contracted Gaussian basis. The usual complete-model-space calculation diverges and a general, incomplete model space is used. Core and core-valence correlation is found to be important, and very good agreement with experimentally known molecular spectroscopic constants is obtained when it is included: equilibrium internuclear separations (except for the very flat x3SIGMA(u)+ state of K2) are accurate to 0.07 angstrom, dissociation and excitation energies to 0.09 eV, and vibrational constants to 6 cm-1.