  Halogen bond (X-bond) is a noncovalent interaction between a halogen atom and
an electron donor. It is often rationalized by a region of the positive
electrostatic potential on the halogen atom, so-called $\sigma$-hole. The
X-bond strength increases with the atomic number of the halogen involved, thus
for heavier halogens, relativistic effects become of concern. This poses a
challenge for the quantum chemical description of X-bonded complexes. To
quantify scalar relativistic effects (SREs) on the interaction energies and
$\sigma$-hole properties, we have performed highly accurate coupled-cluster
calculations at the complete basis set limit of several X-bonded complexes and
their halogenated monomers. The SREs turned to be comparable in magnitude to
the effect of basis set. The nonrelativistic calculations typically
underestimate the attraction by up to 5% or 23% for brominated and iodinated
complexes, respectively. Counter-intuitively, the electron densities at the
bond critical points are larger for SRE-free calculations than for the
relativistic ones. SREs yield smaller, flatter, and more positive
$\sigma$-holes. Finally, we highlight the importance of diffuse functions in
the basis sets and provide quantitative arguments for using basis sets with
pseudopotentials as an affordable alternative to a more rigorous
Douglas-Kroll-Hess relativistic theory.
