  The interaction of tungsten hexacarbonyl W(CO)$_6$ precursor molecules with
SiO$_2$ substrates is investigated by means of density functional theory
calculations with and without inclusion of long range van der Waals
interactions. We consider two different surface models, a fully hydroxylated
and a partially hydroxylated SiO$_2$ surface, corresponding to substrates under
different experimental conditions. For the fully hydroxylated surface we
observe only a weak interaction between the precursor molecule and the
substrate with physisorption of W(CO)$_6$. Inclusion of van der Waals
corrections results in a stabilization of the molecules on this surface, but
does not lead to significant changes in the chemical bonding. In contrast, we
find a spontaneous dissociation of the precursor molecule on the partially
hydroxylated SiO$_2$ surface where chemisorption of a W(CO)$_5$ fragment is
observed upon removal of one of the CO ligands from the precursor molecule.
Irrespective of the hydroxylation, the precursor molecule prefers binding of
more than one of its CO ligands. In the light of these results, implications
for the initial growth stage of tungsten nano-deposits on SiO$_2$ in an
electron beam induced deposition process are discussed.
