An incoming O2 would be easily bounced back by Au(100), in agreement with the well documented observations that Au surfaces are inert to O2. On the other hand, a singlet O2 is more reactive, as expected. On Au(100), the potential energy drops over 1 eV as O2 approaches the surface (Figure ), leading to the chemisorbed singlet O2 (Figure ). With the energy gap between singlet and triplet O2 being also around 1 eV, it indicates that the Au-O bonding energy is just about enough to compensate for the spin transformation from triplet to singlet O2. However, it does not mean that a chemisorbed singlet O2 could easily desorb, which has to proceed along the singlet potential energy surface with a barrier of 1 eV.