Directional Noncovalent Interactions: Repulsion and Dispersion

The interaction energies between
an argon atom and the dihalogens
Br2, BrCl, and BrF have been investigated using frozen
core CCSD­(T)­(fc)/aug-cc-pVQZ calculations as reference values for
other levels of theory. The potential-energy hypersurfaces show two
types of minima: (1) collinear with the dihalogen bond and (2) in
a bridging position. The former represent the most stable minima for
these systems, and their binding energies decrease in the order Br
> Cl > F. Isotropic atom–atom potentials cannot reproduce
this
binding pattern. Of the other levels of theory, CCSD­(T)­(fc)/aug-cc-pVTZ
reproduces the reference data very well, as does MP2­(fc)/aug-cc-pVDZ,
which performs better than MP2 with the larger basis sets (aug-cc-pVQZ
and aug-cc-pvTZ). B3LYP-D3 and M06-2X reproduce the binding patterns
moderately well despite the former using an isotropic dispersion potential
correction. B3LYP-D3­(bj) performs even better. The success of the
B3LYP-D3 methods is because polar flattening of the halogens allows
the argon atom to approach more closely in the direction collinear
with the bond, so that the sum of dispersion potential and repulsion
is still negative at shorter distances than normally possible and
the minimum is deeper at the van der Waals distance. Core polarization
functions in the basis set and including the core orbitals in the
CCSD­(T)­(full) calculations lead to a uniform decrease of approximately
20% in the magnitudes of the calculated interaction energies. The
EXXRPA+@EXX (exact exchange random phase approximation) orbital-dependent
density functional also gives interaction energies that correlate
well with the highest level of theory but are approximately 10% low.
The newly developed EXXRPA+@dRPA functional represents a systematic
improvement on EXXRPA+@EXX.