A molecular mechanics (MM) energy function has been optimized for the seven ab initio hydrogen-bonded structures of the formic acid dimer (FAD). Intermolecular interactions are represented by atomic charges, atomic dipoles, and charge fluxes in addition to the van der Waals terms. Intramolecular force constants are optimized by the analytic SDFF procedure. Ab initio MP2/6-311++G** structures, interaction energies, MP2/6-31+G* normal-mode frequencies, and other properties are very well reproduced, indicating that such a nonbonded interaction model can serve as an appropriate representation of the hydrogen-bonding contribution not only for the FAD but in MM functions of other hydrogen-bonded systems.