A critical note on extrapolated helium pair potentials

It is difficult, if not impossible, to extrapolate the helium pair potential to the limit of a complete basis to within the accuracy needed to improve significantly on existing, directly computed potentials. Even though the basis-set convergence of calculations in a correlation-consistent basis with cardinal number X is dominated by the X−3 term, it is important to account for energy terms that converge more rapidly than ∝X−3. The electron-correlation contribution to the potential will be overestimated noticeably when these terms are not properly taken into account.