text: change in CNT resistivity may then be obtained from the cal- 
  culated coverages and single impurity conductances. 
  
  We ﬁnd that oxidation of the active metal site passivates 
  the sensor in the case of doping by Ti, V, Cr, and Mn un- 
  der standard conditions (room temperature and 1 bar of pres- 
  sure). Among the remaining metals, we identify Ni as is the 
  most promising candidate for CO detection. For this system 
  the change in resistance per active site is generally signiﬁcant 
  (>1 Ω) for small changes in CO concentration in the relevant 
  range of around 0.1–10 ppm. Our approach is quite general 
  and is directly applicable to other nanostructures than CNTs, 
  other functionalizations than metal doping, and other back- 
  grounds than atmospheric air. 
  
  All total energy calculations and structure optimizations 
  have been performed with the real-space density functional 
  theory (DFT) code GPAW [22] which is based on the projector 
  augmented wave method. We use a grid spacing of 0.2 ˚A for 
  representing the density and wave functions and the PBE ex- 
  change correlation functional [23]. Transport calculations for 
  the optimized structures have been performed using the non- 
  equilibrium Green’s function method [24] with an electronic 
  Hamiltonian obtained from the SIESTA code [25] in a dou- 
  ble zeta polarized (DZP) basis set. Spin polarization has been 
  taken into account in all calculations. 