To determine metal binding properties (Figure ), we must first ascertain the charge and spin state of each metal in a given molecule. To do this, we round the atomic partial charge and the atomic partial spin of the metal atom in the molecule to the nearest whole number. These atomic partial charges and spins are obtained from previously-constructed collections in MPcules (see "Atomic Partial Charges" and "Atomic Partial Spins" above). If the rounded charge and spin are incompatible -for instance, if a Mg atom were assigned a charge of +1 and a spin multiplicity of 1 (spin 0) -then the charge is shifted by +1 or -1 (whichever produces a charge which is closer to the calculated atomic partial charge). We shift the charge, rather than the spin multiplicity, because we have found that the spin state of metals is more often well described by DFT than metal charge states (see "Comparison of Atomic Partial Charges and Spin" in the Supplementary Information).