The objective of this work is to obtain a water model for simulations of biological macromolecules in solution. A pragmatic approach is taken in which we use the same type of force field for the water as used for the solute and derive the water potential as an integral part of the ENCAD macromolecular potential.(1,2) Here we describe a flexible three-centered water model (F3C), which has already been used for many large-scale biological simulations, and compare it with other water models. The model is further tested by comparing calculated energetic, structural, and dynamic properties of liquid water, at several temperatures and pressures, with experiment. The F3C model is extremely simple and fits experimental data well for different temperatures, pressures, system sizes, and integration time steps. Because the F3C model works well with short-range truncation, it is well-suited to high-speed computation of long molecular dynamics trajectories of macromolecules in solution.