BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .1. MULTIREFERENCE CONFIGURATION-INTERACTION CALCULATIONS FOR THE 2ND-ROW DIATOMIC HYDRIDES

Multireference configuration interaction calculations (valence electrons only) based on generalized valence bond (GVB) and complete active space (CAS) self-consistent field wave functions are used to compute potential energy functions and spectroscopic constants for the second row diatomic hydrides of aluminum through chlorine. The correlation consistent basis sets of Dunning and co-workers have been used. This suite of sets-standard and augmented sets of double through quintuple zeta quality-provides a systematic means of improving the description of chemical bonding. The regularity of D(e) and r(e) as a function of basis set quality allows extrapolation to an estimated ''complete'' basis set limit. The error in the CAS+1+2 predictions of D(e) for the five species varies from 0.3 kcal/mol (AlH) to 1.4 kcal/mol (HCl) with a root-mean-square (rms) error of 0.7 kcal/mol. The error in r(e) varies from 0.0008 angstrom (SH) to 0.0028 angstrom (SiH) with a rms error of 0.002 angstrom. Other properties are described with comparable accuracy. GVB+1+2 results are only slightly less accurate (rms errors of 1.3 kcal/mol and 0.003 angstrom, respectively). The intrinsic errors inherent in the use of GVB+1+2 and CAS+1+2 wave functions, i.e., the errors at the ''complete'' basis set limit, are found to be much smaller than previously thought. Triple zeta quality sets are found to provide a good compromise between accuracy and cost.