For hydrocarbons, and all two-element molecules, clusters, materials, etc., there are three types of 2-b interactions, which for hydrocarbons are CC, HH, and CH interactions. For 3-b and 4-b there are CCC, CCH, CHH, HHH and CCCC, CCCH, CCHH, CHHH, and HHHH interactions, respectively. We do not consider 5-b interactions here. These interactions are enumerated for all atoms in the molecule. For an n-body interaction among N atoms there are a total of ( N n ) interactions. To limit the number of interactions, physical range cutoffs are used for each n-body interaction and we describe that here.