By noticing that the eigenstates of the HEG Fock matrix are nothing else than pure PWs |G , any HF orbital of a many-electron system can be interpreted as the results of a unitary transformation of the HEG HF problem, so that the same extrapolation law generalizes to single-reference quantum chemical methods of solids and molecules (in the limit of a complete and sufficiently large basis) . In another interpretation, one can assume that the virtual states of very high (continuum) energy lose their molecular character and become closer to PWs, so that their contributions to the correlation energy resemble those of the HEG. Based on that, the same authors proposed a single-point extrapolation of Eq.9, from intermediate points of a single calculation, that converges smoothly and reliably to the basis set limit according to