  Clay minerals are ubiquitous in nature, and the manner in which they interact
with their surroundings has important industrial and environmental
implications. Consequently, a molecular-level understanding of the adsorption
of molecules on clay surfaces is crucial. In this regard computer simulations
play an important role, yet the accuracy of widely used empirical force fields
(FF) and density functional theory (DFT) exchange-correlation functionals is
often unclear in adsorption systems dominated by weak interactions. Herein we
present results from quantum Monte Carlo (QMC) for water and methanol
adsorption on the prototypical clay kaolinite. To the best of our knowledge,
this is the first time QMC has been used to investigate adsorption at a
complex, natural surface such as a clay. As well as being valuable in their own
right, the QMC benchmarks obtained provide reference data against which the
performance of cheaper DFT methods can be tested. Indeed using various DFT
exchange-correlation functionals yields a very broad range of adsorption
energies, and it is unclear a priori which evaluation is better. QMC reveals
that in the systems considered here it is essential to account for van der
Waals (vdW) dispersion forces since this alters both the absolute and relative
adsorption energies of water and methanol. We show, via FF simulations, that
incorrect relative energies can lead to significant changes in the interfacial
densities of water and methanol solutions at the kaolinite interface. Despite
the clear improvements offered by the vdW-corrected and the vdW-inclusive
functionals, absolute adsorption energies are often overestimated, suggesting
that the treatment of vdW forces in DFT is not yet a solved problem.
