Perhaps the most conceptually straightforward approach to QM-based intermolecular force field derivation is to generate many configurations of the system, and fit force field parameters to reproduce the QM energies and/or forces. This approach may be applied to quite large molecules using the fragmentation reconstruction method, but extensive sampling of the intermolecular potential energy surface is required for accurate parameter derivation. Alternatively, ab initio force fields have been developed that break down the QM interaction energy into physically motivated components using intermolecular perturbation theory. These methods incoporate important electronic effects, allow for systematic improvement of intermolecular energies, and can potentially be derived from a very small number of high level ab initio calculations. However, compared with more widely-used transferable force fields, ab initio force fields generally employ a more complex functional form, which is slower to evaluate, and due to the cost of the underlying QM calculation the majority of applications are to relatively small system sizes. In this regard, Grimme's quantum mechanically derived force field (QMDFF) has several advantages. It takes as input only the QM equilibrium structure, partial charges, Hessian matrix, covalent bond orders and semi-empirical torsion scans, and outputs a full molecule-specific force field. The QM input can be relatively cheap, it is has been applied to molecules comprising more than 100 atoms, and can even be used to model bond dissociation and metals. However, it again uses a more complex functional form compared to standard, transferable force fields, and its accuracy in the condensed phase and the feasibility of extending the approach to heterogeneous problems, such as protein-ligand binding, are yet to be established.