The potential energy hypersurfaces (PESs) of heptasulfur (S-7) and of [LiS7](+) have been investigated by ab initio molecular orbital calculations at the G3X(MP2) level of theory. Besides the chairlike seven-membered ring (1a) as the global minimum structure, eight local minimum structures and one transition state have been located on the PES Of S7. The barrier for pseudorotation of 1 a is only 5.6 KJ mol(-1). The boatlike S-7 ring (1b) is 12.1 KJ mol(-1) less stable than la, followed by three isomers of connectivity S-6=S and four open-chain isomers. On the basis of multireference calculations at the MRCI(4,4)+Q/6-31 1G(d) level, the most stable open-chain form of S-7 is a triplet of relative energy 133.1 KJ mol(-1). Thus, the reaction energy (Delta E-0) for the ring opening of 1a is 133.1 KJ mol(-1), halfway between those of the highly symmetrical rings S-6 and S-8. Because of their strong multireference, characters, the stabilities of the biradicalic singlet chains are significantly overestimated by the single-reference-based G3X(MP2) method. The calculated vibrational spectrum of 1 a is in good agreement with experimental data. The various isomers Of S7 form stable complexes with Li+ with coordination numbers of 1-4 for the metal atom and binding energies in the range of -93.8 to -165.7 KJ mol(-1). A total of 15 isomeric complexes have been located, with 13 of them containing cyclic ligands. The global minimum structure (2a) is composed of la, with the Li+ cation linked to the four negatively charged sulfur atoms (symmetry C,). The corresponding complex 2c containing the ligand 1b is by 23.4 KJ mol(-1) less stable than 2a, and a bicyclic crown-shaped LiS7 cation (2e) is by 34.9 KJ mol(-1) less stable than 2a. Even less stable are four complexes with the branched S-6=S ligand. SS bond activation by polarization of the valence electrons takes place on coordination of Li+ to cyclo-S-7 (1a).