Heterogeneous reactions Catalyzed by transition,metal-containing nanoparticles represent a crucial type: of reaction in chemical industry. Because-of the,existing,gap in understanding heterogeneous catalysis between a cluster: of a few atoms and a bulk model of periodic slabs, reactions catalyzed by transition-metal-containing nanoparticles: are still not well understood. Herein; we provide a multiscale modeling approach to study the 'benzene hydrogenation reactions on molybdenum carbide nanoparticles (MCNPs) in the process of hi situ heavy oil upgrading. By coupling the quantum mechanical, (QM) density functional tight-binding (DFTB) method with a molecular mechanical (MM) force field, a QM/MM-model was built to describe the reactants, the nanoparticles and the surroundings: Umbrella sampling (US) was used to calculate the free energy profiles of the benzene hydrogenation reactions in a model aromatic solvent in the in situ heavy oil upgrading conditions. By comparing with the traditional method in computational heterogeneous catalysis, the results reveal new features of the metallic MCNPs.,Rather than being rigid, they are very flexible :under working condition due to the entropic contributions of the MCNPs and the solvent, which greatly affect the free energy profiles of these nanoscale heterogeneous reactions.