We address the diffusion of the oxygen molecule in SiO2, using first-princi
ples spin-polarized total-energy calculations. We find that the potential e
nergy surfaces for the singlet and triplet states are very different in cer
tain regions, and that the O-2 molecule preserves its spin-triplet ground s
tate not only at its most stable interstitial position inside the solid but
also throughout its diffusion pathway. Therefore, the singlet state is not
a good approximation to describe the behavior Of O-2 inside SiO2, and spin
-polarization effects are fundamental to understand the properties of this
system.