The ability of MM to model systems ranging in sizes from thousands to millions of atoms makes it indispensable across a wide range of sciences from biology to materials physics. The key to the general success of MM stems from the force field (FF) and its functional form, which allow the approximate description of the potential energy surface of a system as a simple function of its geometry. Transferable FFs such as GAFF (general AMBER FF), CGENFF (CHARMM general FF) and OPLS-AA are designed to be used in conjunction with their respective highly optimized and benchmarked biological FF counterpart. They are primarily used in simulating drug-like components of systems in, for example, computer-aided drug design, and give non-expert users the ability to parametrize highly diverse expanses of chemical space at very little computational cost. The requirement that a FF be transferable stems from two key points, 1) the parametrization process is a complex and error-prone task that is daunting to the inexperienced user, and 2) an attempt to accurately parametrize all of chemical space would be inconceivable. It is therefore generally assumed that as long as a wide selection of chemical space is covered in the parametrization set then these results can readily be applied to new molecules. Each of the general FFs use libraries composed of thousands of pre-tabulated parameters, intensively fit to experimental and QM data for a set of small molecules that make up their training set. The parametrization goal of these particular FFs focusses on recreating experimental data concerning the condensed phase thermodynamic properties of small organic molecules, such as liquid densities, heats of vaporization and free energies of hydration. This parametrization philosophy follows sound logic as these properties describe the FF's ability to accurately characterize the non-bonded interactions that are also key in protein-ligand binding events. Furthermore, the accuracy and applicability of transferable FFs are aided by efforts such as ForceBalance and the Open Force Field Consortium, which aim to expand the areas of chemical space that can be automatically parametrized via well-documented protocols.