The gas-phase enthalpy needed for ∆H vap is simulated using a single water molecule in the NVT ensemble without periodic boundary conditions. The gas-phase simulation used a Langevin integrator with a time step of 0.5 ns and a collision frequency of 1 ps -1 , a 1.0 ns equilibration period, and a 10.0 ns producion run from which coordinates are saved every 0.1 ps (100,000 frames total). Self-polarization and nuclear quantum effect corrections for ∆H vap and nuclear quantum corrections for c P were applied following previously established procedures. These validation properties were evaluated automatically from the NPT simulations in the course of parameter optimization by setting w (T ) j = 0 in ForceBalance.