Calculated dissociation energies of the alkali metal monoxides NaO and KO

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Abstract
The dissociation energies and the low-lying 2Σ+ -2Π electronic state separations in NaO and KO were calculated with basis sets of approximately TZ quality, at the HF and CISD levels, with both the Davidson and Pople sizeconsistency corrections and the full counterpoise correction for internal basis set superposition error. Particular attention was paid to the effect of internal basis set superposition error (IBSSE) on the calculated properties. Two methods were devised to estimate the part of IBSSE due to the lowering of symmetry in a counterpoise correction with the presence of a ghost centre. Calculated results showed a large IBSSE associated with KO, at the correlated level, which was due to such symmetry lowering in the K+ calculations. For both molecules, the ground state was computed to be a 2Π state, and reasonably good agreement was obtained between the calculated and available experimental values for the 2Σ+ - 2Π separations. The computed values of D 0(NaO, X2Π) and D 0(KO, X2Π) (2·43 and 2·07 eV, respectively) at the highest level of theory used are both lower than the available experimental values derived from electron impact mass spectrometry: reasons for this are suggested.

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