  We present a new analysis of exchange and dispersion effects for calculating
halogen-bonding interactions in a wide variety of complex dimers (69 total)
within the XB18 and XB51 benchmark sets. 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, which
are further confirmed by extensive SAPT analyses. 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.
