Ab initio statistical mechanics of surface adsorption and desorption: I. H$_2$O on MgO (001) at low coverage

TL;DR

This paper introduces a molecular dynamics-based computational scheme to calculate the chemical potential of surface-adsorbed molecules, enabling accurate desorption rate predictions without many prior approximations, demonstrated on water on MgO surfaces.

Contribution

The paper presents a novel ab initio molecular dynamics method to compute the chemical potential of adsorbed molecules at any coverage and temperature, improving accuracy over previous models.

Findings

Calculated an ab initio desorption frequency prefactor over 100 times larger than typical assumptions.

Provided initial ab initio estimates of water adsorption energy on MgO (001).

Suggested that PBE functional may underestimate adsorption energies, indicating need for further studies.

Abstract

We present a general computational scheme based on molecular dynamics (m.d.) simulation for calculating the chemical potential of adsorbed molecules in thermal equilibrium on the surface of a material. The scheme is based on the calculation of the mean force in m.d. simulations in which the height of a chosen molecule above the surface is constrained, and subsequent integration of the mean force to obtain the potential of mean force and hence the chemical potential. The scheme is valid at any coverage and temperature, so that in principle it allows the calculation of the chemical potential as a function of coverage and temperature. It avoids all statistical mechanical approximations, except for the use of classical statistical mechanics for the nuclei, and assumes nothing in advance about the adsorption sites. From the chemical potential, the absolute desorption rate of the molecules can be computed, provided the equilibration rate on the surface is faster than the desorption rate. We apply the theory by {\em ab initio} m.d. simulation to the case of H$_2$O on MgO (001) in the low-coverage limit, using the Perdew-Burke-Ernzerhof (PBE) form of exchange-correlation. The calculations yield an {\em ab initio} value of the Polanyi-Wigner frequency prefactor, which is more than two orders of magnitude greater than the value of $10^{13}$ s$^{-1}$ often assumed in the past. Provisional comparison with experiment suggests that the PBE adsorption energy may be too low, but the extension of the calculations to higher coverages is needed before firm conclusions can be drawn. The possibility of including quantum nuclear effects by using path-integral simulations is noted.