The interaction of hydrogen atoms and molecules with a silica surface is relevant for many research and technological areas. Here, the dynamics of hydrogen atoms colliding with an H-preadsorbed b-cristobalite (001) surface has been studied using a semiclassical collisional method in conjunction with a recently developed analytical potential energy surface based on Density Functional Theory (DFT) calculations. The atomic recombination probability via an Eley-Rideal (E-R) mechanism as well as the probabilities for other competitive molecular surface processes have been determined in a broad range of collision energies (0.04-3.0) eV eV) for off-normal (q v =45°) and normal (q v =0°) incidence and for two different surface temperatures (T S = 300 and 1000 K). H 2,gas molecules form in roto-vibrational excited levels while the energy transferred to the solid surface is below of 10% for all simulated conditions. Finally, the global atomic recombination coefficient (gE-R) and vibrational state resolved recombination coefficients g(v)were calculated and compared with the available experimental values. The calculated collisional data are of interest in chemical kinetics studies and fluid dynamics simulations of silica surface processes in H-based low-temperature, low-pressure plasmas.

We present a method to study hydrodynamic phenomena from atomistic simulations. In statistical mechanics, these fields are computed as the ensemble average over the time dependent probability density function corresponding to the time evolution of an initial conditional probability density function consistent with some initial conditions. These initial conditions typically consist in constraints on some macroscopic fields, e.g. the density field. We show how these processes can be studied by combining the dynamical approach to non-equilibrium molecular dynamics with the restrained simulation approach. As an illustration of our method, we study the relaxation to the equilibrium of an interface between two immiscible liquids. We show that, at a variance with the local time average method, the standard atomistic approach used in this field, our method is able to produce (macroscopic) fields satisfying the symmetry conditions of the problem.

We introduce a combined Restrained MD/Parallel Tempering approach to study the difference in free energy as a function of a set of collective variables between two states in presence of unknown slow degrees of freedom.We applied this method to study the relative stability of the amorphous vs crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function of the temperature. We found that, at variance with bulk systems, at low T small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase transition at higher T . On the contrary, large nanoparticles recover the bulk-like behavior: crystalline at low T and amorphous at high T .