C. Core-level spectroscopy with DFT/CIS Conventional TD-DFT can be used to describe the x-ray near-edge region, involving transitions from core MOs to the lowest unoccupied MO (LUMO) and other low-lying virtual orbitals. However, core excitation energies are systematically underestimated (i.e., insufficiently bound), when computed using either generalized gradient approximations (GGAs) or hybrid functionals. Errors increase with the atomic number of the element in question. One way of tackling this problem is simply to shift the calculated TD-DFT spectra, since the relative energies of different spectral features are much more accurate than the absolute excitation energies, although this can be problematic in systems with multiple overlapping elemental edges, especially where heavier elements are involved. Adjusting the fraction of exact exchange in the XC functional can reduce the need for empirical shifting, although large fractions ( 50% Hartree-Fock exchange) are often necessary. To avoid this, Besley and co-workers parameterized SRC1 and related functionals specifically for core-level spectroscopy. These functionals use range separation, in conjunction with the short-range µBLYP exchange functional, to apply a large fraction of exact exchange on a length scale of < 1 Å. The SRC functionals accurately reproduce elemental K-edge excitation energies but perform quite poorly in ∆SCF calculations, suggesting that their success rests mostly on error cancellation.