UC Riverside

The treatment of atomic anions with Kohn–Sham density functional theory (DFT) has long been controversial because the highest occupied molecular orbital (HOMO) energy, EHOMO, is often calculated to be positive with most approximate density functionals. In Chapter 1, we assess the accuracy of orbital energies and electron affinities for all three rows of elements in the periodic table (H–Ar) using a variety of theoretical approaches and customized basis sets. Among all of the theoretical methods studied here, we find that a nonempirically tuned range-separated approach provides the best accuracy for a variety of basis sets, even for small basis sets where most functionals typically fail. While previous approaches utilize non-self-consistent methods, the nonempirically tuned range-separated procedure used here yields well-defined electronic couplings/gradients and correct EHOMO values because both the potential and resulting electronic energy are computed self-consistently. Orbital energies and electron affinities are further analyzed in the context of the electronic energy as a function of electronic number (including fractional numbers of electrons) to provide a stringent assessment of self-interaction errors for these complex anion systems. In Chapter 2, we present a new analysis of exchange and dispersion effects for calculating halogen-bonding interactions in a wide variety of complex dimers (69 total). Contrary to previous work on these systems, we find that dispersion plays a more significant role than exact exchange in accurately calculating halogen-bonding interaction energies. In particular, we find that even if the amount of exact exchange is non-empirically tuned to satisfy known DFT constraints, we still observe an overall improvement in predicting dissociation energies when dispersion corrections are applied, in stark contrast to previous studies (J. Chem. Theory Comput. 2013, 9, 1918-1931). In addition to these new analyses, we correct several (14) inconsistencies in the "XB51" set, which is widely used in the scientific literature for developing and benchmarking various DFT methods. Together, these new analyses and revised benchmarks emphasize the importance of dispersion and provide corrected reference values that are essential for developing/parameterizing new DFT functionals specifically for complex halogen-bonding interactions.