A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolarizable potential functions

The ability of simple potential functions to reproduce accurately the densi ty of liquid water from -37 to 100 degrees C at 1 to 10 000 atm has been fu rther explored. The result is the five-site TIP5P model, which yields signi ficantly improved results; the average error in the density over the 100 de grees temperature range from -37.5 to 62.5 degrees C at 1 atm is only 0.006 g cm(-3). Classical Monte Carlo statistical mechanics calculations have be en performed to optimize the parameters, especially the position of the neg ative charges along the lone-pair directions. Initial calculations with 216 molecules in the NPT ensemble at 1 atm focused on finding a model that rep roduced the shape of the liquid density curve as a function of temperature. Calculations performed for 512 molecules with the final TIP5P model demons trate that the density maximum near 4 degrees C at 1 atm is reproduced, whi le high-quality structural and thermodynamic results are maintained. Attain ment of high precision for the low-temperature runs required sampling for m ore than 1 billion Monte Carlo configurations. In addition, the dielectric constant was computed from the response to an applied electric field; the r esult is 81.5 +/- 1.5 at 25 degrees C and the experimental curve is mirrore d from 0-100 degrees C at 1 atm. The TIP5P model is also found to perform w ell as a function of pressure; the density of liquid water at 25 degrees C is reproduced with an average error of similar to 2% 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. (C) 2000 American I nstitute of Physics. [S0021-9606(00)50820-4].