Two more terms in eq. 2.2 can be accurately formulated: the electron-nuclear attraction (𝑉 ne , called the 'external potential' in DFT language) and the classical Coulomb interaction between the electrons (𝐽 ee ). The only terms to be approximated now are: the difference between the kinetic energy of the KS system and the actual real system (𝑇 -𝑇 KS ) and the electron-electron interaction terms which arise from the quantum nature of electrons: the "exchange" term (𝐸 𝑋 ) as a result of the Pauli principle and the "Coulomb" (𝐸 𝐶 ) term resulting from the fact that electrons are not point charges (which is captured by 𝐽) but have a probability distribution in space which depends on all other electrons. These last three terms are summed up in the exchange-correlation functional (𝐸 𝑋𝐶 in eq. 2.2). DFT is an exact theory up to this point and the approximations arise since the exact form for 𝐸 𝑋𝐶 is unknown (it is even "unknowable" ). Furthermore, although DFT is a theory based on electron density, orbitals are reintroduced in the KS-approach (which underlies the vast majority of all published DFT data). Although this introduces a degree of arbitrariness which was heavily discussed in the literature previously, it allows to use the molecular orbital picture for interpretation of DFT results as done for HF theory. In practice, KS-and HF-orbitals have very similar shapes and lead to similar bonding interpretation. More care has to be taken when discussing the meaning of (virtual) orbital energies.