which are replaced by Fast Fourier Transforms (FFTs) in the case of discrete representation of the ϕ i . In principle, because the charge density ρ depends on the square of the orbitals, the maximum radius for the density expansion in the reciprocal space should be as high as 2G ϕ max , meaning that a density cutoff energy E ρ cut = 4E ϕ cut (c.f. Eq.5) is needed to maintain a consistent resolution between orbitals and densities in reciprocal space . For example, a factor 4 is used here by default in the calculation of the exchange and Coulomb integrals involved in the zero-order HF calculation. For the MP2 correlation energy (Eq.2), whatever the ratio between E if the pair densities ρ ia (G) are precalculated and stored in memory. Consequently, the number N ρ ia G of G vectors entering the expansion of ρ ia has a drastic effect on the overall performance of the MP2 energy evaluation as it also affects the prefactor and memory requirements. For this reason, it is imperative to study below to what extent a reduction in E ρ ia cut alters the accuracy of the MP2 energy (Section IV A).