  We use DFT to study the effect of molecular adsorbates on the conductance of
metallic carbon nanotubes. The five molecules considered (NO2, NH2, H, COOH,
OH) lead to similar scattering of the electrons. The adsorption of a single
molecule suppresses one of the two available channels of the CNT at low bias
conductance. If more molecules are adsorbed on the same sublattice, the
remaining open channel can be blocked or not, depending on the relative
position of the adsorbates. If a simple geometric condition is fulfilled this
channel is still open, even after adsorbing an arbitrary number of molecules.
