Density functional calculations with simulated annealing have been performed for cyclo-heptasulfur dioxide, S7O2, which is the only stable homocyclic dioxide of sulfur that has been prepared in a pure state. The relatively flat energy surface allows substantial motion of the atoms, and there are numerous structures corresponding to local minima. The calculations show that (1) exocyclic S-O bonds are favored over endocyclic S-O-S bridge bonds, (2) S7-chair structures are favored over "boat" structures, (3) relative to the S7 Unit, axial SO bonds are favored over equatorial and cis-SO bonds are favored over trans, and (4) singlet structures are favored over triplet. We discuss the bonding patterns and the consequences for other cyclic sulfur oxides.