New Empirical Procedures for Improving ab Initio Energetics

Empirical schemes are developed for correcting the energies resulting from a moderately high-level ab initio calculation. Each method involves only a few adjustable parameters, which are determined by fitting a set of 31 high-spin atomization energies. Parameters are developed only for elements through the 2 p block. The methods are based upon (1) bond lengths, (2) the electron densities at mid-bond critical points, (3) correlation energy scaling, (4) basis-set extrapolation, and (5) the number and spin of the valence electrons. A hybrid method, in which correlation energy is scaled by a factor that depends on the mid-bond electron densities, is also investigated. Each type of correction achieves a marked improvement over ab initio atomization energies; the first two are especially effective. Additional expressions are developed to represent the expected (1σ) uncertainties for predicted atomization energies.