Ground and excited states of K2 and K2+ by the open-shell coupled cluster method

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Abstract
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 K2 + 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.