Juxtaposition of spectra from two different basis sets in Fig. makes it clear that basis-set effects on the O(1s) → LUMO feature at 532 eV are rather small, consistent with the testing reported in Section 3 B. In contrast, the N(1s) → continuum transitions are extremely sensitive to the choice of basis, which is a general characteristic of continuum states in Gaussian basis sets. At the SRC1-r1/6-311++G(d,p) level there is a continuum feature just below the oxygen K-edge that is significantly more intense than the O(1s) → LUMO transition, and the two features might easily be confused, whereas at the SRC1-r1/def2-TZVPD level the N(1s) → continuum transitions manifest as a small pre-edge feature. In view of these and other artifacts, and in the absence of filtering, there is no way to be confident in the assignment of the K-edge transition, given the large peak shifts that often characterize XAS calculations using TDDFT. 4.C. Chemically relevant examples. Having seen that dipole filtering is essential to remove continuum artifacts in broadband TDKS spectra, we next discuss some chemically-motivated examples where unfiltered spectra (in realistic basis sets) obscure key information. 4.C.1. Ionic liquid ion pair. Our first example is an ion-pair dimer, 1-butyl-3-methylimidazolium (C 4 C 1 Im + ) and thiocyanate (SCN -), representing a typical dialkylimidazolium ionic liquid. This and other roomtemperature ionic liquids have been using NEXAFS spectroscopy at the nitrogen and sulfur K-edges. In ac- The complete TDKS spectrum ("full") is compared to filtered spectra involving only the O(1s), N(1s), or S(2p) orbitals, as indicated. The spectra have been shifted vertically for clarity but they share a common intensity scale such that the sum of the filtered spectra equals the full spectrum. which follows from Eq. ( ).