O2 molecule is a diradical with its ground state being a triplet. When an O2 is put close to Au(100), DFT calculations could lead to artificial change in the spin and the triplet state of O2 could not be conserved. Geometric optimization would produce the structure shown in Figure , with O2 in the singlet state. Such an O2* is only 0.16 eV lower in energy than a triplet O2 far away from Au(100), in agreement with previous reports. However, it would be incorrect to attribute this structure to physisorption, just based on its low adsorption energy. The Au-O distance is around 2.2 Å, comparable to the Au-O distance of 2.01~2.07 Å in Au2O3 crystal. The Bader charge on O2* is at -0.5e, while the O-O distance upon adsorption is elongated to 1.33 Å, a significant increase from the O-O distance of either triplet O2 (1.21 Å) or singlet O2 1.23 Å. In other words, it's actually a chemisorbed structure, with Au-O bonding interactions, superoxide O-O bond and significant negative charge on O2*. A more realistic evaluation of the interaction between O2 and Au(100) is obtained by spinpolarized calculations with spin restriction to force the system to stay either as a triplet or a singlet.