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

Multireference configuration interaction calculations (valence electro ns only) based on generalized valence bond (GVB) and complete active s pace (CAS) self-consistent field wave functions are used to compute po tential energy functions and spectroscopic constants for the second ro w diatomic hydrides of aluminum through chlorine. The correlation cons istent 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 ch emical bonding. The regularity of D(e) and r(e) as a function of basis set quality allows extrapolation to an estimated ''complete'' basis s et limit. The error in the CAS+1+2 predictions of D(e) for the five sp ecies 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 fro m 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 kc al/mol and 0.003 angstrom, respectively). The intrinsic errors inheren t in the use of GVB+1+2 and CAS+1+2 wave functions, i.e., the errors a t the ''complete'' basis set limit, are found to be much smaller than previously thought. Triple zeta quality sets are found to provide a go od compromise between accuracy and cost.