Full configuration interaction benchmark calculations of first-order one-electron properties of BH and HF

Full configuration interaction benchmark calculations have been carried out for the electric dipole and quadrupole moments and the electric field gradient at the nuclei of BH and HF. The accuracy of perturbation theory from second to fourth order and coupled cluster theory with up to triple excitations has been investigated. For all the properties the coupled cluster models outperform the perturbation series. The convergence of the series of coupled cluster models is significantly faster and more systematic than the one of the perturbation series, and only the coupled cluster series defines a hierarchy of models with well defined levels of accuracy. The CCSD(T) model is a good approximation to the full coupled cluster singles, doubles, and triples model. It recovers 80%-90% of the full effect of triple excitations, and the small error due to the approximate description of triple excitations is comparable in size to the error due to neglect of higher-order excitations. For accurate calculations, the CCSD(T) model is the preferred model for all the properties. Our estimates of the exact dipole moments mu(e(HF))=1.802+/-0.003 D and mu(0(BH))=1.356+/-0.004 D are in perfect agreement with the experimental values of mu(e(HF))=1.803+/-0.002 D and mu(0(BH))=1.27+/-0.21 D. (C) 1999 American Institute of Physics. [S0021-9606(99)30502-X].