Thus, the signature of these approaches is that no proper excited-state calculation is performed, but instead the requisite information for XAS is extracted from groundstate molecular orbitals (MOs), albeit possibly involving promotion of a fractional electron, as described below. This description encompasses a variety of eigenvaluebased approaches include Slater's transition method (STM) and its generalizations, along with the transition potential method (TPM) and its generalizations. (The TPM can be viewed as an approximation to Slater's original idea.) Some other approaches such as the full core-hole method (FCHM) fall under this umbrella as well. Some of these methods are based on the use of fractional-electron selfconsistent field (SCF) calculations, and they differ in whether the virtual orbitals are probed one by one, as in Slater's original conception, or whether only the lowest unoccupied MO (LUMO) is used. The latter approach is the basis of TPM and its variants, which afford an entire core-level spectrum (at one particular edge) from a single calculation. These methods have the same computational cost as the ∆SCF approach, in which a full electron is promoted into the virtual space in a state-by-state manner, but without the need for stateby-state calculations. They may hold some advantages for modeling complex systems or experiments insofar as the spectrum is computed in a single shot and is closely tied to initial-state Kohn-Sham eigenvalue information.