Testing Semiempirical Quantum Mechanical Methods on a Data Set of Interaction Energies Mapping Repulsive Contacts in Organic Molecules

Semiempirical
quantum mechanical (QM) methods with corrections
for noncovalent interactions provide a favorable combination of accuracy
and computational efficiency that makes them a useful tool for a study
of large molecular systems. It was, however, noted that the accuracy
of these methods deteriorates at intermolecular distances shorter
than equilibrium. In this work, we explore this issue systematically
using a newly developed data set of benchmark interaction energies
named R160×6. This data set maps repulsive contacts in organic
molecules, and it consists of 160 model complexes for which six points
along the dissociation curve are provided. Testing a wide range of
semiempirical QM methods against the CCSD­(T)/CBS benchmark revealed
that most methods, and all the dispersion-corrected ones, underestimate
the repulsion systematically. The worst cases are usually hydrogen–hydrogen
contacts. The best results were obtained with PM6-D3H4 and DFTB3-D3H4,
as these methods already contain a correction for the H–H repulsion,
but the errors are still about twice as large as in equilibrium geometries.