The parameters I Z and A Z are the first vertical ionization potential and electron affinity, respectively, and E H Z and E L Z can be related to the corresponding energies of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals according to the Koopmans and Janak theorem . In addition to reaction energies for chemical half reactions (Eqn. ( )), it is also possible to evaluate full reaction energies including all relevant forward and backward steps .