ABINITIO COUPLED-CLUSTER CALCULATIONS OF ISOTROPIC HYPERFINE SPLITTING IN SOME DIATOMIC HYDRIDES

Ab initio calculations of the isotropic hyperfine coupling constants in the electronic ground states of some diatomic hydride radicals and their radical cations are reported using highly correlated wave functions. An augmented coupled-cluster doubles procedure, including an approximate account of the effect of the inclusion of amplitudes due to single and triple replacements in the unrestricted Hartree-Fock (UHF) reference determinant, is employed. Contact spin densities are estimated by finite-field perturbation theory. Coupling at oxygen in OH and OH+ and at fluorine in FH+ is well described by a basis set of triple-ζ quality including two shells of polarization (d) functions. Accurate computation of the nitrogen splitting in NH and NH+ and particularly the carbon splitting in CH appears to be more demanding. A modest double-ζ basis set on hydrogen is sufficient to quantitatively reproduce the observed trends in the coupling constants at that nucleus. In contrast to previous work with correlated calculations employing spin-restricted reference spaces, in the UHF-based approach used here it is shown that freezing the core electrons produces only relatively small changes in the computed isotropic splitting. © 1990 American Chemical Society.