We chose features based on the chemical and physical properties that affect pK a . The key properties chemists are trained to think about in relation to ionization are the ionizable atom, resonance, inductive effects, steric effects, and solvation. These properties affect the ability of an ionizable group to support a protonated or deprotonated state along with the associated change in formal charge. We also considered the quantum mechanical approach for calculating pK a using a thermodynamic cycle involving the gas phase acidity and the Fig. We use the partial charge on the deprotonated atom (yellow) and the average partial charge on atoms one bond (purple) and two bonds (orange) away from that atom in both the protonated and deprotonated form of the molecule, making a total of six features involving partial charges.