A study of the effect of anharmonicities and large amplitude vibrations on the thermodynamic properties of the water dimer is presented. Different vibrational models have been constructed using ab initio data obtained at the MP2(Full)/6-311++G(2d,2p) level. In particular, we present the first complete analysis of the rotation of the hydrogen donor monomer around the O ... O axis. The potential barrier was found to be 221 cm(-1). A variational calculation of the torsional energy levels yields a fundamental frequency of 105 cm(-1) The O ... O stretching mode is described using a Morse function. The fundamental frequency and the dimerization energy are calculated to be 153 cm(-1) and 5.15 kcal/mol, respectively, in agreement with the experimental results. For the dimerization reaction we have calculated Delta S, Delta H, and the equilibrium constant, K-p. The results show that inclusion of anharmonicity into the vibration modes favors the lower experimental limit for Delta S and the upper limit for Delta H. In addition, the anharmonic corrections reduce the difference between calculated and experimental K-p. This difference decreases with temperature. A high-temperature limit of 3.47 x 10(-5)atm(-1) was found for K-p.