GerritJan Linker

Koopmans' theorem IP ≈ ε_{i} , 1 ≤ i ≤ N EA ≈ ε_{a} , N+1 ≤ a N is the number of electrons. IP = Ionisation Potential = the amount of energy needed to remove an electron from the molecule EA = Electron Affinity = the amount of energy needed to remove an electron from the charged negative ion. The IP is equal to ε_{HOMO} of the uncharged molecule. The EA is equal to ε_{HOMO} of the singly charged molecule. Two errors in the calculation of Koopman's IP's work to (partially) cancel eachother. Orbital relaxation and correlation. Orbitals are not relaxed after extracting the electron. Also correlation effect are not taken into account. Occupied HF orbital energies are self interaction free: The electrons in an occupied HF orbital feels the potential of N1 electrons. Unoccupied HF orbital energies are not selfinteraction free: The electrons in virtual HF orbitals feel the potential of N electrons. Binding energy (Koopmans) BE_{i}(KT)=E_{i}^{(N1)}(FO)E^{N}(HF) = ε_{i} KT = Koopmans Theorem ; FO = Frozen Orbital ; HF = Hartree Fock
