tomistic models are the most detailed models used in simulation of all but the smallest molecules. Here molecules are represented as a number of atomic sites connected by chemical bonds. The interaction between these atoms is described by a potential, commonly known as a force field, which includes terms to describe bond stretches, bond angle bends, torsional rotations and non-bonded interactions. Additional terms to describe coupling between different distortions (e.g. coupling between a torsional rotation and a bond stretch) or other interactions such as hydrogen bonding may also be present. The exact form of each term can vary from force field to force field. Bond stretches are usually described by a Taylor expansion of the energy about the equilibrium bond length