Halogen Bonds: Benchmarks and Theoretical Analysis

We
carried out an extensive survey of wave function and DFT methods
to test their accuracy on geometries and dissociation energies of
halogen bonds (XB). For that purpose, we built two benchmark sets
(XB18 and XB51). Between the DFT methods, it was found that functionals
with high exact exchange or long-range corrections were suitable for
these dimers, especially M06-2X, ωB97XD, and double hybrids.
Dispersion corrections tend to be detrimental, in spite of the fact
that XB is considered a noncovalent interaction. Wave function techniques
require heavy correlated methods (i.e., CCSD­(T)) or parametrized ones
(SCS-MP2 or SCS­(MI)­MP2). Heavy basis sets are needed to obtain high
accuracy, such as aVQZ or aVTZ+CP, and ideally a CBS extrapolation.
Relativistic ECPs are also important, even for the bromine based dimers.
In addition, we explored some XB with new theoretical tools, the NCI
(“Non-Covalent Interactions”) method and the NOFF (“Natural
Orbital Fukui Functions”).