Energy Surfaces and Structure of S₇O

The geometries corresponding to local minima in the energy surfaces of cycloheptasulfur monoxide, S₇O, have been determined by the combined molecular dynamics/density functional (MD/DF) method. Beginning from the ground-state structure of S₈, we show that the ring structure (i.e. with a-S-O-S-bridge bond) is a stable local minimum in the energy surface. Applying kinetic energy corresponding to a temperature of 2000 K to the atoms triggers a chemical rearrangement: The O atom moves out of the ring and forms a double bond outside an S₇ ring. The parameter-free MD/DF scheme allows us to follow the evolution of the structure and, on cooling, to find other stable minima. The lowest energy we find (~0.1 eV below the ring structure) is in remarkably good agreement with experimental data. The results provide a striking demonstration of the value of the MD/DF technique in a system where the energy surfaces have many local minima and the ground state has a geometry qualitatively different from the initial one.