Unlinked cluster effects in molecular electronic structure. I. The HCN and HNC molecules

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Abstract
Extensive calculations on the molecules HCN and HNC have been performed using our recently proposed ’’coupled‐pair approximation’’ (CPA) in a basis of nonorthogonal independent‐pair natural orbitals. A number of linear geometries are used for both systems, allowing prediction of equilibrium geometry, rotational constants and force constants for the stretching vibrational modes. The CPA values are in substantially better agreement with experimental results (where available) than those obtained from variational CI calculations including all double excitations, and can be generated with little extra computational effort. In addition, several approximate coupled‐pair techniques, which require no more effort than a CI calculation, are investigated in order to estimate their accuracy relative to the full coupled‐pair method. Using the bond‐stretching potentials, we have calculated vibrational energy levels and transition energies. Again, the values obtained by the CPA method are in better agreement with experiment than the CI results—vibrational transitions are predicted to about 90 cm−1 (3%). Inclusion of the unlinked cluster terms is shown to be essential for this agreement, and the use of the CPA in the nonorthogonal IPNO basis results in a compact and powerful formalism within which all important dynamical correlation effects may be calculated.