Method to Calculate the Entropy of Liquids Deriving a liquid's partition function and entropy by directly integrating the Boltzmann factor over the high-dimensional potential energy surface is an intractable challenge. Mean-field approaches solve this problem by drastically reducing the dimensionality. A method is presented to calculate the effective potential of a molecule in a liquid from the forces measured in a computer simulation. In the case of liquid argon, a one-dimensional harmonic potential gives quantitative agreement with perturbation calculations and experiment. Liquid water requires six harmonic potentials for the three hindered translations and rotations, the latter being derived from the molecular torques. There are additional entropic terms to account for the variable number of water's hydrogen bonds and the directionality of donors and acceptors. As well as giving good agreement with perturbation calculations and experiment, the method provides an insightful decomposition of water's entropy. The entropy of the transition state for hydrogen-bond switching in water is also calculated, yielding the rate constant for that process.