Kantonen et al have used a similar QM-to-MM parameter mapping approach. They derive Lennard-Jones parameters from the MBIS partitioned atomic electron densities, with two fitting parameters (used to map the electron density decay constants onto the Lennard-Jones σ and ) for each of C, O, N, H, and polar H. These QM-derived Lennard-Jones parameters are used alongside the AM1-BCC charge model, and GAFF bonded parameters. On a test set of 23 small organic molecules, they report MUEs of 0.027 g/cm 3 and 1.1 kcal/mol in density and heat of vaporisation, respectively, which is similar in performance to our model 5d protocol. Visscher and Geerke have used an AIM-based QM-to-MM mapping scheme, using both C 6 and C 8 dispersion coefficients, in combination with an ESP-based charge scheme and a charge-on-spring polarisable model. Our strategy of fitting a small number of R f ree i parameters was also employed there, but no off-site charges were used.