The interaction energies of over a thousand water dimer configurations have been calculated using the symmetry-adapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a site-site form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available. (C) 1997 American Institute of Physics.