High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S-6. Twelve isomeric structures of S-6 have been identified: two unbranched rings (chair and boat), one trigonal prism of D-3h symmetry, two singly branched rings (S-5=S), three triplet chains, one singlet chain, and three doubly branched rings (S=S-4=). The prism structure is essentially a cluster of three S-2 molecules connected via a six-center pi*-pi*-pi* interaction. It is by 51 kJ mol(-1) less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S-6, which requires an activation energy of 149 kJ mol(-1). The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S-2 in chemical reactions involving elemental sulfur. (C) 2004 American Institute of Physics.