Binding in Radical-Solvent Binary Complexes: Benchmark Energies and Performance of Approximate Methods

In many situations, weak interactions between radicals and their environment potentially influence their properties and reactivity. We computed benchmark binding energies of 12 binary complexes involving radicals, using basis set extrapolated coupled cluster theory with up to CCSDT(Q) excitations plus corrections for core correlation and relativistic effects. The set was comprised of both electron-rich and electron-poor small radicals which were either neutral or positively charged. The radicals were complexed with the closed-shell polar (model) solvent molecules H2O and HF. On the basis of these accurate ab initio binding energies, we assess the performance of many modern DFT functionals for these radical-solvent molecule interactions. Radical hydrogen bonded complexes are well-described by most DFT methods, but two-center-three-electron interactions are at least slightly overbound by most functionals evaluated here, including range-separated functionals. No such systematic error was found for electron-rich metal-water complexes. None of the functionals tested yield chemical accuracy for all types of complexes.