The σ and parameters for molecular-mechanical force fields are typically assigned by performing simulations of bulk liquids using various parameter sets. For each set of parameters, properties like the density and enthalpy of vaporization of the neat liquid are calculated. The parameters that yield the most accurate properties are used as the standard Lennard-Jones potential for that atom type. While this practice has been effective, there are several associated drawbacks. This procedure assumes the Lennard-Jones parameters are transferable to atoms of the same type in other molecules. Additionally, fitting parameters to properties of bulk liquids becomes more difficult for polyatomic molecules because the Lennard-Jones parameters for multiple atom types must be fit simultaneously. Both and σ are treated as free parameters of empirical force fields, along with hundreds of other parameters. This creates the possibility that the parameterization procedure will generate unphysical values for and σ.