meaning that a different fractional occupancy (n i = 2/3) is used, as compared to Slater's original approach. We will refer to both of these approximations as STMs, and they are collected in Table along with some other approximations that are introduced below. What Eqs. ( ) and ( ) share in common is that either method requires only a single (fractional-electron) SCF calculation to estimate BE i . In contrast, the generalized (G)STM approximations that are introduced below each require two or more SCF calculations. This additional complexity may be warranted if the accuracy improves with respect to the corresponding ∆SCF calculation.