In addition, we investigate the question of whether to include Lennard-Jones parameters on polar hydrogen atoms, or to set them to zero and effectively absorb them onto the neighbouring heavy atom (Section S1.3). In Table , we show that force field protocol 4a, which has no Lennard-Jones parameters on polar hydrogens (denoted 'H0'), has a similar density error, but higher heat of vaporisation error (0.90 kcal/mol) compared to the other protocols. Interestingly, this model has a much lower R f ree i for nitrogen than the other models, but the low density error indicates that this does not propagate through to too short hydrogen bonding distances in condensed phase simulations. Nevertheless, we conclude that explicitly including Lennard-Jones parameters on polar hydrogen atoms has accuracy benefits, and we continue to use this approach in the following force field models.