Force fields contain the parameters used for a particular functional form of the molecular potential energy, and are a keystone in many areas of chemical research ranging from medicine to materials research. There are two primary concerns with building and using force fields. The first is the physical model which are the physical parameters used in the potential energy, such as a particular bond length parameter. The second is the chemical model, sometimes called the perception model, which determines how to map the physical parameters onto the molecule. Most force field implementations perform both tasks, and examples are GAFF 1 , CGenFF 2 , and ATB . These take as input a molecule and as output the paa) Electronic mail: dmobley@uci.edu rameters that match the molecule's particular chemical environments.