Classical molecular mechanics force fields are widely used approximations to recover the energy and forces of an atomistic system as a function of its nuclear coordinates. Force fields are an invaluable companion to quantum mechanical modelling in cases where the number of atoms or the time scale to be modelled would be otherwise restricted. Such techniques have seen applications in, for example, the modelling of battery materials, organic light-emitting diodes, crystal structure prediction, and particularly in computeraided drug design where they form the basis of molecular dynamics, docking and free energy simulations for calculating protein-ligand binding affinity. Typically, for modelling organic molecules in biology and chemistry, the force field is a sum of bonded (including harmonic bond-stretching and angle-bending terms, and anharmonic 4-body torsion potentials) and non-bonded (including Coulombic and Lennard-Jones interactions) terms. Small molecule force fields of this form include GAFF, CGenFF, OPLS, and the Open Force Field 1.0.0 ('Parsley') force field. In general, the bonded and charge parameters of the force field may be fit to quantum mechanical potential energy surfaces and electrostatic potentials, respectively. On the other hand, the Lennard-Jones parameters are nearly always fit to experimental pure liquid properties. To be broadly effective, small molecule force fields require accurate parameterisation of each of the above terms for all chemical space. While recent trends have seen parameters fit to larger datasets, complete coverage is challenging. Numerous studies, such as the SAMPL blind challenges have shown that there is plenty of room for improvement when it comes to predictive molecular modelling with force fields.