To reduce the number of parameters needed to define the force field, each atom in the system is assigned an atom type. All atoms of the same type are assumed to have the same Lennard-Jones parameters. The type of an atom is generally specified by its element, hybridization, and bonding partners. The number and definition of atom types varies widely between different force fields. For instance, for the molecules studied in this paper, the CGenFF, GAFF, and OPLS force fields have 67, 32, and 153 atom types, respectively.