Class 1 IPs such as OPLS 9 contain only harmonic bonded interactions with periodic torsions (Equation ) and have mainly been parameterized for small organic molecules. Class 2 IPs such as PCFF+ and COMPASS 11 add additional interactions (e.g. anharmonicity and cross-terms for covalent interactions as well as a softer stopping potential for the Lennard-Jones interaction) and have been typically applied polymer systems.