In the triplet state, a physisorbed O2 is indeed optimized, as shown in Figure . Furthermore, without spin transformation, a triplet O2 approaching Au(100) passes through a flat energy surface into a shallow physisorption well, as shown in Figure . Here, it is instructive to compare the cases of Si(100) and Au(100). On Si(100), a triplet O2 can have significant bonding interaction with Si, which traps O2 on the surface and guarantees its eventual spin transformation to the energetically much more favorable singlet state. On Au(100), an adsorbed triplet O2 is actually slightly more stable than an adsorbed singlet O2 (although only by about 0.007 eV), and without bonding interaction between triplet O2 and Au atoms the probability for spin transformation is low.