A parameter-free density functional that works for noncovalent interactions

The failure of semilocal density functional theory for medium- and long-range noncovalent molecular interactions is a long-standing challenge for computational chemistry. Here, we assess the performance of the random phase approximation (RPA), a parameter-free fifth-rung functional, for reaction energies governed by changes in medium- and long-range noncovalent interactions. Our benchmark data include relative energies of alkane isomers, two sets of isomerization reactions testing intramolecular dispersion, a set of dimers of biological importance, a hierarchy of n-homodesmotic reactions, and the predissociation of a ruthenium-based Grubbs catalyst with bulky ligands. The RPA results are an order of magnitude more accurate than those of popular semilocal functionals such as PBE or B3LYP and more systematic than those of semiempirical functionals parametrized for weak interactions, such as B2PLYP-D or M06-2X. In conclusion, RPA is highly promising for thermochemical applications, particularly if noncovalent interactions are important.