Application of an approximate double substitution coupled cluster (ACCD) method to the potential curves of CO and NeHe: Higher order correlation effects in chemically and weakly bonded molecules

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Abstract
The recently introduced approximate double substitution coupled cluster (ACCD) method has been applied to two diatomics: CO and NeHe. The ACCD method, using an electron pair matrix operator approach, is computationally efficient, has the orbital invariance properties of the complete CCD treatment, and goes to the proper pair description in the limit of separated electron pairs. ACCD and CCD results for the He2 potential curve are virtually identical. The calculations on CO and NeHe are among the first tests of ACCD for more than a four‐electron system. Application to these two diatomics provides an opportunity to compare higher order correlation effects in chemically bonded and weakly attractive species. It is found that ACCD agrees quite well with CCD for both systems. However, the higher order correlation effects seem much less important in NeHe than in CO, where CCD reduces the error in the prediction of the vibrational frequency by a factor of 3 over a variational treatment that accounted only for singly and doubly substituted correlating configurations.