An improved fully flexible fixed-point charges model for water has been developed to predict the vapor-liquid coexistence properties using the NVT-Gibbs ensemble Monte Carlo technique (GEMC) and the pressure in supercritical region. The average deviation between our simulation and experimental data for saturated liquid densities is 2.75% over temperature range of 314-609 K. Comparing with experimental data for T,, P-c and p(c) (P-c = 220.064 bar, T-c = 647.096 K and p(c) = 0.322 g/cm(3) for the experimental data), our calculated results (Pc = 213 bar, T, = 644.3 K and pc = 0.325 g/cm(3) for our simulations) are acceptable and are better than those by the SPC-E and TlP4P models. The saturated pressure is calculated by evaluating the pressure of vapor from NPT-MD simulation at the coexistence vapor densities at the nominal temperature. The agreement of our simulated pressures of supercritical water at any density and temperature with the experimental values is excellent. The second virial coefficient and radial distribution function in ambient and supercritical conditions are also estimated. The radial distributions consist with experimental data very well. (c) 2007 Elsevier B.V. All rights reserved.