Unlinked cluster effects in molecular electronic structure. III. Potential curve for the CN− ion and the adiabatic electron affinity of CN

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Abstract
A ground state potential curve (around equilibrium) is constructed for the CN− ion, using CI and coupled‐pair methods. Significant changes arise in the values of the spectroscopic constants when the effects of unlinked cluster excitations are included; a set of predicted CN− spectroscopic constants is obtained using the uncertainties observed in a recent calculation on HCN. Evidence of the great similarity in the HCN, HNC, and CN− electronic structures is obtained by analyzing the CN− valence‐shell correlation energy into pair contributions, thus giving a very simple picture of the correlation effects. Using the direct CI method extended to include approximate coupled pair techniques, the ground state energy for the CN radical is calculated, obtaining the adiabatic electron affinity of CN to form CN−; agreement with the very accurate experimental data is satisfactory.