The potential energy surfaces of organic molecules for practical applications are typically modelled using transferable molecular mechanics force fields such as AMBER, CHARMM, GROMOS 10 or OPLS. When combined with molecular dynamics (MD) or Monte Carlo (MC) sampling, these force fields may be used to predict, for example, liquid properties of small molecules, structural propensities of peptides, and protein-ligand binding free energies, all with reasonable accuracy. The intramolecular component of the force field is typically modelled by harmonic bond and angle potentials to represent two-and three-body terms, respectively, an anharmonic torsional term to model dihedral rotations, and Coulomb and Lennard-Jones terms to account for interactions between atoms separated by three or more bonds. Details vary between these transferable force fields, but the fixed functional form is common to all. Thus, no matter how carefully the force field is parametrized, accuracy will ultimately be limited by the choice of this functional form.