In this study, the DMC calculations were carried out for water monomer, dimer, trimer, and three isomers of the water hexamer (prism, book, and cage). The ZPEs of the dimer and trimer are subsequently used to compute the dissociation energies of the dimer and trimer, while the calculations for the hexamer are mainly exploratory. In these DMC calculations, the imaginary time step ∆τ = 5 a.u. and α = 0.1 are used. For monomer, dimer and trimer, five DMC calculations were performed for each system. In each DMC calculation, the number of walkers is 50 000, and these walkers are equilibrated for 5000 time steps followed by 50 000 propagation steps. The statistical uncertainty is estimated as the standard deviation of the 5 DMC runs for the same system.