  We propose a practical approach to spin-polarized systems within the van der
Waals density functional (vdW-DF). The method was applied to a gas phase oxygen
molecule and a parallel (H-type) pair of oxygen molecules. It was found that
vdW-DF improves the equilibrium distance and binding energy. In particular, one
type of vdW-DF can describe such systems reasonably well. The van der Waals
interaction has been confirmed to have an energy comparable to the magnetic
one, while emerging at a distance rather longer than the latter.
