Notably, the force fields used in the simulations mentioned above are usually transferable force fields such as OPLS (optimised potentials for liquid simulations), GAFF (general Amber force field) and UFF (universal force field). These force fields share similar functional forms, and their parameters are usually trained to reproduce QM or experimental properties of small organic molecules. A common underlying approximation of these force fields is that their libraries of the atom, bond, angle and torsion types are transferable and can be applied to molecules outside the training set. Moreover, they are mostly designed for simulating small molecules in a biological context. We note two major problems associated with applying these general-purpose force fields to OLED material simulations. Firstly, not all the atom, bond, angle and torsion types present in the OLED molecules are included in these force fields, in which case partial refit or parameter tweaking is required. 18 Also, since the atom types do not change upon electronic excitation, neither do the bonded parameters, limiting the ability of the force field to account for geometric changes (for example, bond elongation or torsional rotation) in the excited state. Secondly, the parameters themselves in these force fields correspond to the ground state only, whereas, in OLED research, electronic excited states (for example, low lying singlet (S 1 ) and triplet (T 1 ) states) are often just as relevant, in particular for the calculation of emission properties. While it is possible to extract force field charges from quantities such as the electronic excited state density or potential, 15 there is no guarantee that these will be compatible with the rest of the force field. This is particularly problematic for the development of Lennard-Jones parameters suitable for electronic excited states, as they are highly interdependent on the force field charges and usually rely on experimental data, such as liquid density, for their fitting.