In recent years, we have been following a similar strategy to Grimme's QMDFF, focusing more on condensed phase properties and heterogeneous systems. The basis of this approach is the density derived electrostatic and chemical (DDEC) atoms-in-molecule (AIM) scheme, which partitions the total electron density into approximately-spherical atomcentered basins. Atomic charges are derived by integrating the atomic electron density over all space and, in contrast to direct fitting of the QM electrostatic potential (ESP charges), it is possible to derive chemically-meaningful DDEC atomic electron densities and charges for both surface and buried atoms. A further advantage of this approach is that the Lennard-Jones parameters may also be computed directly from the atomic electron densities, using methods based on the Tkatcheko-Scheffler relations that are commonly used to incorporate dispersion effects into density functional theory (DFT) calculations. Similar to the Grimme approach, these non-bonded parameters are derived from a single QM optimized structure, which would be problematic if the charges show strong conformation dependence. However, Manz and Sholl have demonstrated that DDEC charges are transferable between different conformations of a molecule (as measured by their ability to recreate the QM electrostatic potential), and they conclude that the charges are suitable for the construction of flexible force fields. Furthermore, it should be noted that atoms-in-molecule electron density partitioning lends itself naturally to the derivation of both off-site charges to model electron anisotropy (such as lone pairs and σ-holes) and atomic polarizabilities, though we have not yet investigated a fully polarizable force field.