CoachElectron correlation is the part of the electrons’ mutual avoidance not described by a single determinant (whose electrons of opposite spins move independently in each other’s average field). The correlation energy is the exact energy minus the Hartree–Fock energy in a complete basis; it is always negative.
Examples
Example 3.18 (Ionising )
At , the STO-3G orbital energy of is : Koopmans predicts , against the measured . The frozen orbitals overestimate the energy of the ion (it would relax), the missing correlation acts the other way; for valence ionisations the two errors largely cancel, which is why the photoelectron spectra of the Year 2 volume can be read with orbital diagrams.