Coordination number. Water models published after 2000 exhibited a fairly narrow range of values for the average number of water molecules in the first coordination shell of a given water molecule, with a minimum value of 4.6 for the TIP3P-ST water model, and a maximum of 5.2 for the SWM4-NDP polarizable water model (Figure ). We chose an experimental value of 4.7 for comparison, which was based on X-ray scattering data and supported by molecular dynamics simulations with a polarizable force field. 121 The PCFF water model exhibited the greatest deviation from the experimental value, with an average of 13.5 ± 0.4 water molecules in the first coordination shell. Indeed, the O-O RDF for PCFF is significantly different from the other water models (Figure , Supporting Information), with a much broader peak and first minimum at 4.6 Å, compared to ~3.3-3.4 Å for most other water models. As the coordination number is found by integrating to the first minimum in the RDF, this creates a significant difference in the calculated coordination number for the water molecule. However, despite this large difference, the O-O, O-H, and H-H RDFs for PCFF are in general qualitatively similar to the other models. The PCFF force field, as part of the CFF force field family, 129 was parameterized to reproduce the structure and vibrational frequencies of isolated molecules in the gas phase, 128 not bulk properties, which may account for some of the observed discrepancies. The original SPC and TIP3P-based models (including CVFF) also produced larger coordination numbers compared to the experimental value. The SPC/E model provided greater agreement compared to SPC by changing the partial charges only, while TIP3P-Ew improved agreement by changing both partial charges and Lennard-Jones parameters. The SPC/Fw provided greater agreement for flexible models by small changes to the Lennard-Jones parameters and changes to the water geometry, while retaining the original SPC partial charges.