It is important to notice explicitly that the exchange term, which is always positive for local spin-orbitals, is absent in simple Hartree product. Hence the electron-electron repulsive energy ⟨ ⟨ Ψ Ψ 0 | G 2 | Ψ Ψ 0 ⟩ ⟩ on the antisymmetrized product of spin-orbitals is always lower than the electron-electron repulsive energy on the simple Hartree product of the same spin-orbitals. Since exchange bielectronic integrals are different from zero only for spin-orbitals with parallel spins, we link the decrease in energy with the physical fact that electrons with parallel spin are kept apart in real space in Slater determinant states.