Scaling of correlation basis set extension energies

A simple formula for extrapolating energies calculated from finite basis sets to the limit of a complete basis set by scaling the basis set extension energies is presented. Combining this basis set extrapolation method with the scaling-all-correlation (SAC) energy method, it is possible to extrapolate correlated electronic structure calculations to the limit of full dynamical correlation with a complete basis set. Based on this combined scaling method, two variants of G3 theory, and G3SCB and G3SCB(MP3), are proposed, with the total mean unsigned errors (MUEs) comparing favourably with those of G3 and G3(MP3).