We report refined results for the equilibrium constant for water dimerization (K-P), computed as a function of temperature via fully coupled 6D calculation of the canonical (H2O)(2) partition function on VRT(ASP-W)III, the most accurate water dimer potential energy surface currently available. Partial pressure isotherms calculated for a range of temperatures and relative humidities indicate that water dimers can exist in sufficient concentrations (e.g., 10(18) m(-3) at 30 degreesC and 100% relative humidity) to affect physical and chemical processes in the atmosphere. The determinations of additional thermodynamic properties (DeltaG, DeltaH, DeltaS, C-P, and C-V) for (H2O)(2) are presented, and the role of quasi-bound states in the calculation of K-P is discussed at length.