A hierarchy of electronic structure calculations, scalings, and fittings were used to develop an analytic intermolecular potential for dimethyl methylphosphonate (DMMP) interacting with the TiO2 rutile (110) surface. The MP2/aug-cc-pVDZ (6-311+G* for Ti) level of theory, with basis set superposition error (BSSE) corrections, was used to calculate multiple intermolecular potential curves between TiO5H6 as a model for the Ti and O atoms of the TiO2 surface, and CH3OH and O - P(CH3)(OH)2 as models for different types of atoms comprising DMMP. Each intermolecular potential energy emphasized a particular atom-atom interaction, and the curves were fit simultaneously by a sum of two-body potentials between the atoms of the two interacting molecules. The resulting analytic intermolecular potential gives DMMP/TiO5H 6 potential curves in excellent agreement with those calculated using MP2/aug-cc-pVDZ (6-311+G* for Ti) theory. MP2 theory with the smaller basis set, 6-31++G* (6-31G* for Ti), gives DMMP/TiO 5H6 potential energy curves similar to those found using MP2/aug-cc-pVDZ (6-311+G* for Ti), suggesting the smaller basis set may be used to describe DMMP interactions with larger cluster models of the TiO 2 surface. The TiO5H6 cluster does not model either the 6-fold coordinated Ti atoms or the bridging O atoms of the TiO 2 (110) surface, and to also model these atoms MP2/6-31++G* (6-31G* for Ti) theory was used to calculate potential energy curves for DMMP interacting with the larger Ti3O13H14 cluster and much large cluster Ti11O40H36 cluster. The two-body potential energy curves for DMMP/TiO5H 6 were scaled to fit both the DMMP/Ti3O13H 14 and DMMP/Ti11O40H36 potential energy curves. The resulting parameters for the 5- and 6-fold coordinated Ti atoms and bridging and bulk O atoms were used to develop an analytic intermolecular potential for DMMP interacting with rutile TiO2 (110).