The mapping parameters (two per element) are fit to experimental liquid properties using the ForceBalance software. Our own work has focused on the development of a QUantum mechanical BEspoke (QUBE) force field and associated toolkit (QUBEKit), built on QM-to-MM parameter mapping. QUBE bond and angle force field parameters are derived from the QM Hessian matrix of the molecule under study, using the modified Seminario method. Atomic partial charges are computed from the density derived electrostatic and chemical (DDEC) partitioned atomic electron densities. The Tkatchenko-Scheffler method is used to derive C 6 parameters from the same atomic electron densities, and the repulsive part of the Lennard-Jones potential is derived from atoms-in-molecule atomic radii. Once all other parameters are in place, flexible torsion parameters can be fit to QM dihedral scans using interfaces between QUBEKit and external QM and MM software packages. A small number of mapping parameters (in this case free atom radii for use in the derivation of Lennard-Jones parameters) is used to ensure accuracy of condensed phase properties, and the resulting force fields have also been shown to perform well in the calculation of protein-ligand binding free energies. Force field design choices can be tested and optimised.