  We report here how the hydration of complex surfaces can be efficiently
studied thanks to recent advances in classical molecular density functional
theory. This is illustrated on the example of the pyrophylite clay. After
presenting the most recent advances, we show that the strength of this implicit
method is that (i) it is in quantitative or semi-quantitative agreement with
reference all-atoms simulations (molecular dynamics here) for both the
solvation structure and energetics, and that (ii) the computational cost is two
to three orders of magnitude less than in explicit methods. The method remains
imperfect, in that it locally overestimates the polarization of water close to
hydrophylic sites of the clay. The high numerical efficiency of the method is
illustrated and exploited to carry a systematic study of the electrostatic and
van der Waals components of the surface-solvant interactions within the most
popular force field for clays, CLAYFF. Hydration structure and energetics are
found to weakly depend upon the electrostatics. We conclude on the consequences
of such findings in future force-field development.
