  Water molecules adsorbed on inorganic substrates play an important role in
several technological applications. In the presence of light atoms in
adsorbates, nuclear quantum effects (NQE) influence properties of these
systems. In this work, we explore the impact of NQE on the dissociation of
water wires on stepped Pt(221) surfaces. By performing ab initio molecular
dynamics simulations with van der Waals corrected density functional theory, we
note that several competing minima for both intact and dissociated structures
are accessible at finite temperatures, making it important to assess whether
harmonic estimates of the quantum free energy are sufficient to determine the
relative stability of the different states. We perform ab initio path integral
molecular dynamics (PIMD) in order to calculate these contributions taking into
account conformational entropy and anharmonicities at finite temperatures. We
propose that when when adsorption is weak and NQE on the substrate are
negligible, PIMD simulations can be performed through a simple partition of the
system, resulting in considerable computational savings. We calculate the
contribution of NQE to the free energies, including anharmonic terms. We find
that they result in an increase of up to 20% of the quantum contribution to the
dissociation free energy compared to harmonic estimates. We also find that the
dissociation has a negligible contribution from tunneling, but is dominated by
ZPE, which can enhance the rate by three orders of magnitude. Finally we
highlight how both temperature and NQE indirectly impact dipoles and the
redistribution of electron density, causing work function to changes of up to
0.4 eV with respect to static estimates. This quantitative determination of the
change in work function provides a possible approach to determine
experimentally the most stable configurations of water oligomers on the stepped
surfaces.
