The calculation results show that H2O monomer prefers to adsorb at two top sites (t1, t2) of the icosahedral Ru-55 nanoparticle. When two H2O molecules coadsorb on Ru-55, they choose to adsorb at the two adjacent vertex and edge sites to form a H2O dimer. The energy barrier for H2O monomer dissociation is 0.43 eV on t1 site and 0.34 eV on t2 site after DFT-D2 correction, indicating the t2 site is more active than the t1 site. The energy barrier for H2O dimer dissociation on t2 site (2H(2)O ->(TS1HOHH2O)-H-step1) is only 0.11 eV after DFT-D2 correction. During H2O dimer dissociation, the H atom from the H2O locating on the t1 site can automatically transfer to the H2O locating on the more active t2 site. The low energy barriers for the dissociation of H2O monomer and dimer illustrate that Ru-55 nano particles have a high catalyst activity for H2O dissociation to produce hydrogen. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.