ENTROPY-DRIVEN STRUCTURES OF THE WATER OCTAMER

Ab initio Hartree-Fock and Moller-Plesset second-order perturbation ca lculations have been performed on several structures of the water octa mer which are expected to be in low-lying energy states. The ab initio results calculated with double zeta plus polarization basis sets pred ict that the global minimum energy octamer has a cubical structure wit h D2d symmetry at 0 K, while the cubical structure with S4 symmetry is more stable than the D2d Structure above almost-equal-to 40 K. Howeve r, above almost-equal-to 230 K, the cyclic octamers with S8 and S4 Sym metries are more stable than the cubical species because of the entrop y effect.