We have used diffusion Monte Carlo (DMC) simulations to calculate the energy barrier for H$_2$ dissociation on the Mg(0001) surface. The calculations employ pseudopotentials and systematically improvable B-spline basis sets to expand the single parti
cle orbitals used to construct the trial wavefunctions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 550 atoms, show that all these errors together can be reduced to $sim 0.03$ eV. The DMC dissociation barrier is calculated to be $1.18 pm 0.03$ eV, and is compared to those obtained with density functional theory using various exchange-correlation functionals, with values ranging between 0.44 and 1.07 eV.
We have used diffusion Monte Carlo (DMC) calculations to study the structural properties of magnesium hydride (MgH$_2$), including the pressure-volume equation of state, the cohesive energy and the enthalpy of formation from magnesium bulk and hydrog
en gas. The calculations employ pseudopotentials and B-spline basis sets to expand the single particle orbitals used to construct the trial wavefunctions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 1050 atoms, show that all these errors together can be reduced to below 10 meV per formula unit. We find excellent agreement with the experiments for the equilibrium volume of both the Mg and the MgH$_2$ crystals. The cohesive energy of the Mg crystal is found to be 1.51(1) eV, and agrees perfectly with the experimental value of 1.51 eV. The enthalpy of formation of MgH$_2$ from Mg bulk and H$_2$ gas is found to be $0.85 pm 0.01$ eV/formula unit, or $82 pm 1$ kJ/mole, which is off the experimental one of $76.1 pm 1$ kJ/mole only by 6 kJ/mole. This shows that DMC can almost achieve chemical accuracy (1 kcal/mole) on this system. Density functional theory errors are shown to be much larger, and depend strongly on the functional employed.
It is well known, both theoretically and experimentally, that alloying MgH$_2$ with transition elements can significantly improve the thermodynamic and kinetic properties for H$_2$ desorption, as well as the H$_2$ intake by Mg bulk. Here we present a
density functional theory investigation of hydrogen dissociation and surface diffusion over Ni-doped surface, and compare the findings to previously investigated Ti-doped Mg(0001) and pure Mg(0001) surfaces. Our results show that the energy barrier for hydrogen dissociation on the pure Mg(0001) surface is high, while it is small/null when Ni/Ti are added to the surface as dopants. We find that the binding energy of the two H atoms near the dissociation site is high on Ti, effectively impeding diffusion away from the Ti site. By contrast, we find that on Ni the energy barrier for diffusion is much reduced. Therefore, although both Ti and Ni promote H$_2$ dissociation, only Ni appears to be a good catalyst for Mg hydrogenation, allowing diffusion away from the catalytic sites. Experimental results corroborate these theoretical findings, i.e. faster hydrogenation of the Ni doped Mg sample as opposed to the reference Mg or Ti doped Mg.
