The parts of GMTKN55 that are dedicated to intermolecular and noncovalent interactions are not used here as this would mostly be testing the dispersion correction, in our view. However, we will examine interaction energies for a set of F -(H 2 O) n clusters where ionwater delocalization error could be problematic. For that system, we extracted cluster configurations from a classical molecular dynamics simulation, which were then trimmed to contain n = 3-7 water molecules. Benchmark interaction energies for removing the ion, while keeping the (H 2 O) n geometry fixed, were computed at the level of second-order Møller-Plesset perturbation theory (MP2) within the resolution-of-identity (RI) approximation. To obtain complete basis-set RI-MP2 estimates, we combined the HF/aug-cc-pVQZ energy with a two-point extrapolation of the RI-MP2 correlation energy, 127 using aug-cc-pVTZ and aug-cc-pVQZ. 128,129 2.1.2. TD-DFT Benchmarks. For valence excitation energies we employ a data set consisting of 23 local excitations and 8 CT excitations that was assembled by Tozer and co-workers (Table ). Geometries for this data set can be found elsewhere. For core-excited transition energies at the carbon, nitrogen, and oxygen K-edges, we use a benchmark data set from Ref. 131 (Table ), with geometries from Ref. 66. All of these benchmark calculations employ full linear response in the TD-DFT calculations, meaning that the Tamm-Dancoff approximation (TDA) is not invoked. This helps to identify the transitions for which reference data are available, by comparison to previously published benchmarks. Core-tovalence excitation energies are computed using the core/ valence separation (CVS) technique. A data set containing 22 donor-acceptor complexes was used to compute S 1 excitation energies having CT character. Benchmark excitation energies for this data set were computed for the present work, using configuration interaction singles corrected for double excitations, [CIS(D)]. 136 (This method has elsewhere been used to benchmark CT excitation energies. 137 ) Reference values computed at the CIS(D)/aug-cc-pVTZ level can be found in Table . For TD-DFT calculations on this data set, we do employ the TDA.