At E ϕ cut (N G ) sufficiently large, N vir is large enough to recover the CBS energy and ε n acts as the cutoff energy of an auxiliary basis which is gradually expanding towards the CBS limit. The fact that the orbitals and eigenvalues of Eq.9 originate from the complete basis has no effect on the extrapolation in practice. This technique has the advantage of considerably truncating the virtual space required to calculate the MP2 correlation energy since a maximum n of the order of n max = 10 000-20 000 is satisfactory for extrapolating relative energies. In addition, the Fock operator must be diagonalized only for the n max orbitals of lowest eigenvalues .