  Protoplanetary disks are the target of many chemical studies (both
observational and theoretical) as they contain the building material for
planets. Their large vertical and radial gradients in density and temperature
make them challenging objects for chemical models. In the outer part of these
disks, the large densities and low temperatures provide a particular
environment where the binding of species onto the dust grains can be very
efficient and can affect the gas-phase chemical composition.
  We attempt to quantify to what extent the vertical abundance profiles and the
integrated column densities of molecules predicted by a detailed gas-grain code
are affected by the treatment of the molecular hydrogen physisorption at the
surface of the grains. We performed three different models using the Nautilus
gas-grain code. One model uses a H2 binding energy on the surface of water (440
K) and produces strong sticking of H2. Another model uses a small binding
energy of 23 K (as if there were already a monolayer of H2), and the sticking
of H$_2$ is almost negligible. Finally, the remaining model is an intermediate
solution known as the encounter desorption mechanism. We show that the
efficiency of molecular hydrogen binding (and thus its abundance at the surface
of the grains) can have a quantitative effect on the predicted column densities
in the gas phase of major species such as CO, CS, CN, and HCN.
