MBIS is similar in idea to the Hirshfeld family of AIM methods. In this method the total electron density is partitioned onto a minimal set of s-type Slater functions, whose parameters are fit to the input molecular electron density by minimising the Kullback-Leibler divergence. Like DDEC, it has been shown that the MBIS-derived charges reproduce the QM ESP to good accuracy and are robust to small conformational changes, while MBIS can also be used to derive Tkatchenko-Scheffler based dispersion coefficients. MBIS charges, along with all AIM properties (atomic multipoles up to quadrupole order and radial moments) that we require for force field derivation have recently been implemented in the PSI4 quantum chemistry package, which we can now access through our latest interface with QCengine. Model 3b thus investigates the use of MBIS as the AIM partitioning method in force field derivation. For technical reasons, it was necessary to use the B3LYP exchange-correlation functional, and the IEFPCM implicit solvent model (with a chloroform solvent to mimic a dielectric of approximately 4), for the underlying QM calculations. Again this combination of methods gave very good accuracy on our training set, with errors in the density and heat of vaporisation of 0.016 g/cm 3 and 0.58 kcal/mol, respectively.