Slater's transition state concept can be used to compute electron BEs in a manner that relies on MO energy levels ε i rather than a difference of total SCF energies, requiring only a single SCF calculation per BE. As such, we omit N from the notation in Eq. ( ) and let E denote the ground-state energy of the initial state. Slater considered this energy to be a continuous function of the MO occupation numbers n i . We follow a slightly different formulation, taking E(q) to be a function of a single continuous variable q, equal to the fraction of an electron that is removed from whichever MO is to be ionized. [The index of this MO, corresponding to i in Eq. (1), will be implicit.] The energy required to completely ionize this MO can be expressed as 58