  Van der Waals (vdW) forces play a fundamental role in the structure and
behavior of diverse systems. Thanks to development of functionals that include
non-local correlation, it is possible to study the effects of vdW interactions
in systems of industrial and tribological interest. Here we simulated within
the framework of density functional theory (DFT) the adsorption of isooctane
(2,2,4-trimethylpentane) and ethanol on a Fe(100) surface, employing various
exchange-correlation functionals to take vdW forces into account. In
particular, this paper discusses the effect of vdW forces on the magnitude of
adsorption energies, equilibrium geometries and their role in the binding
mechanism. According to our calculations, vdW interactions increase the
adsorption energies and reduce the equilibrium distances. Nevertheless, they do
not influence the spatial configuration of the adsorbed molecules. Their effect
on the electronic density is a non-isotropic, delocalized accumulation of
charge between the molecule and the slab. In conclusion, vdW forces are
essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.
