Thermochemical data from ab initio calculations. I. Estimation of SCF energies for augmented basis sets and Hartree–Fock limit energies

A procedure is outlined which allows an estimation of molecular energies both for a finite basis set including polarization functions and for the Hartree–Fock limit. It is shown that the orbital error of a given minimal basis is covered to a certain relatively constant percentage by an augmented basis set calculation. Thus an improvement factor Q av can be determined by analyzing the corresponding results of small molecules where reasonable estimates of HF limit energies can be taken from the literature. For a combination of Pople's STO ‐3G and 6‐31G* basis sets Q av turns out to be 0.955.