Computing Reaction Kinetics with MC-PDFT–OPESf: Combining Multireference Electronic Structure Theory and Enhanced Sampling

Accurate rate constants are crucial for understanding and optimizing catalytic reactions mediated by enzymes, metalloproteins, and heterogeneous catalysts. These systems frequently present a dual computational challenge. Multiconfigurational reaction sites require multireference techniques for the accurate treatment of the electronic structure, and high activation barriers prevent efficient sampling of unbiased reactive transitions. In this work, we combine multiconfiguration pair-density functional theory (MC-PDFT) as an accurate and efficient multireference electronic structure method with on-the-fly probability-enhanced sampling flooding (OPESf) as an enhanced sampling method capable of accelerating reactive transitions. We demonstrate the approach on the Diels-Alder [4+2] cycloaddition between <i>cis</i>-butadiene and ethene as a reaction characterized by a large activation barrier and multireference character. MC-PDFT-OPESf provides reaction rates in agreement with experiments at a fraction of the computational cost required by conventional unbiased <i>ab initio</i> calculations. We propose MC-PDFT-OPESf as an efficient approach for computing kinetics in strongly correlated molecular systems.