A Molecular “Thermometer” for Measuring Effective Non‐Local Exchange

ABSTRACT Non‐local exchange (NLX) is a key ingredient for accurate density functional calculations, but its effective strength is often hard to quantify beyond simple global hybrids. To this end, we introduce a molecular probe based on the isomerization of hexaethynylbenzene to carbo ‐benzene, a reaction with exceptional sensitivity to exchange effects. Using the isomerization energy, we define a simple relative measure, , that gauges the effective NLX of a method on a scale ranging from 0 for the local density approximation to 100 for Hartree‐Fock. Across a broad set of density functional approximations, closely tracks the formal Hartree‐Fock exchange content of global hybrids, while revealing that most common GGAs, meta‐GGAs, and global hybrids still provide too little effective exchange. Several range‐separated hybrid, double hybrid, and some local hybrid functionals perform much better and closely approach the near‐basis‐set‐limit coupled‐cluster reference value of . The values of various semiempirical quantum mechanical and machine‐learned interatomic potential methods indicate that they empirically account for NLX effects to a varying degree. Thus, the proposed reaction serves as a compact thermochemical benchmark for assessing and developing methods that aim to reliably describe NLX.