PNO–CI and CEPA studies of electron correlation effects. III. Spectroscopic constants and dipole moment functions for the ground states of the first-row and second-row diatomic hydrides

A systematic investigation of the ground state potential curves and dipole moment functions has been performed for the diatomic hydrides LiH to HCl on the basis of variational configuration interaction wavefunctions PNO–CI and the coupled electron pair approximation CEPA. The basis sets of Gaussian-type orbitals are derived by simple rules from optimized atomic sets available in the literature. Between 95% (LiH) and 85% (HCl) of the valence shell correlation energies are accounted for in the CEPA calculations. Core–valence intershell correlation has been included for several members of each row, and its effect on the potential curves is analyzed. Comparison of the spectroscopic constants derived from the CEPA potential curves with experiment shows a high reliability of the theoretical values. The standard deviations over both rows are as follows: re:0.003 Å, ωe:14 cm−1, αe:0.005 cm−1, and ωexe:1.5 cm−1. The errors of the calculated dissociation energies go up to 0.3 eV but behave very regularly. They are believed to allow for predictions which are correct to about ±0.05 eV. The following new D0 values are recommended (empirical values in parenthesis): NH:3.40 (3.2±0.16); NaH:1.88 (2.05±0.2); MgH:1.23 (2.0±0.5); PH:3.02 (3.5±0.3). Various vibrational matrix elements have been calculated from the dipole moment curves. The μ0 values show errors of 0.02–0.04 D. The differences between diagonal elements as well as the transition elements for the first few levels deviate by only about 10% from the available experimental data.