  In this work, second-generation Car-Parrinello-based QM/MM molecular dynamics
simulations of small nanoparticles of NbP, NbAs, TaAs and 1T-TaS$_2$ in water
are presented. The first three materials are topological Weyl semimetals, which
were recently discovered to be active catalysts in photocatalytic water
splitting. The aim of this research was to correlate potential differences in
the water structure in the vicinity of the nanoparticle surface with the
photocatalytic activity of these materials in light induced proton reduction.
The results presented herein allow to explain the catalytic activity of these
Weyl semimetals: the most active material, NbP, exhibits a particularly low
water coordination near the surface of the nanoparticle, whereas for
1T-TaS$_2$, with the lowest catalytic activity, the water structure at the
surface is most ordered. In addition, the photocatalytic activity of several
organic and metalorganic photosensitizers in the hydrogen evolution reaction
was experimentally investigated with NbP as proton reduction catalyst.
Unexpectedly, the charge of the photosensitizer plays a decisive role for the
photocatalytic performance.
