An intermolecular potential for a formaldehyde dimer that includes many body effects was constructed from the monomer properties and intermolecular perturbation theory calculations. The interaction energy was calculated as a sum of various terms with physical significance. A global minimum of C-s symmetry similar to that arrived at from experimental calculations was predicted for the dimer; its calculated energy, -3.82 kcal/mol, is comparable to that provided by ab initio MP2/6-31G++(2d,2p) calculations (-3.54 kcal/mol). Stationary points for the trimer were preliminarily explored; the geometries obtained contained a variable number of hydrogen bonds and exhibited differences in the nature of their interactions, A molecular dynamics method was used to simulate the Liquid phase of formaldehyde; a first-coordination sphere containing about 13 molecules that remained quite structured even at fairly long distances was obtained.