Ground and excited states of K₂ and K₂⁺ 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≤R≤7 Å and the three lowest states of K ₂⁺ at 2≤R≤15 Å, 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 x ³Σ _u⁺ state of K₂) are accurate to 0.07 Å, dissociation and excitation energies to 0.09 eV, and vibrational constants to 6 cm^-1.