In order to properly describe reactions in heterogeneous catalyst systems, the reactants, solvent, and bulk effects of the surface must be taken into account. Embedded-cluster QM (quantum mechanics)/MM (molecular mechanics) methods can treat reactions on surfaces (the gas−surface interface), and the effective fragment potential method (EFP) can accurately treat the solvent effects on reactions (the gas−liquid interface). In order to create a QM/MM/EFP hybrid method for treatment of heterogeneous catalytic systems in the presence of a solvent (the liquid−surface interface), an EFP−MM interaction potential has been developed. Example calculations on small clusters of silica and water have been carried out.