The ability of simple potential functions to reproduce accurately the density of liquid water from Ϫ37 to 100°C at 1 to 10 000 atm has been further explored. The result is the five-site TIP5P model, which yields significantly improved results; the average error in the density over the 100°t emperature range from Ϫ37.5 to 62.5°C at 1 atm is only 0.006 g cm Ϫ3 . Classical Monte Carlo statistical mechanics calculations have been performed to optimize the parameters, especially the position of the

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... egative charges along the lone-pair directions. Initial calculations with 216 molecules in the NPT ensemble at 1 atm focused on finding a model that reproduced the shape of the liquid density curve as a function of temperature. Calculations performed for 512 molecules with the final TIP5P model demonstrate that the density maximum near 4°C at 1 atm is reproduced, while high-quality structural and thermodynamic results are maintained. Attainment of high precision for the low-temperature runs required sampling for more than 1 billion Monte Carlo configurations. In addition, the dielectric constant was computed from the response to an applied electric field; the result is 81.5Ϯ1.5 at 25°C and the experimental curve is mirrored from 0-100°C at 1 atm. The TIP5P model is also found to perform well as a function of pressure; the density of liquid water at 25°C is reproduced with an average error of ϳ2% over the range from 1 to 10 000 atm, and the shift of the temperature of maximum density to lower temperature with increasing pressure is also obtained.