Koopmans' theorem [59] can be understood as follows: for
closed-shell systems, the negative of the HOMO energy is the ionization
potential. That is, the energy required to form the cation provided that
the ionization process is adequately represented by the removal of an electron
from an orbital without change in the wave-functions of the other electrons.

The only alternative way to calculate the I.P. is to calculate the
ΔH_{f}_{ } of the parent species, then, without allowing the geometry to relax,
calculate the
ΔH_{f}_{ } of the ionized system. The difference in
ΔH_{f}_{ },
in kcal.mol^{-1}, divided by 23.06, is the predicted I.P., in eV.