Structures of water octamers (H2O)8: exploration on ab initio potential energy surfaces by the scaled hypersphere search method.

The potential energy surface (PES) of water octamers has been explored by the scaled hypersphere search method. Among 164 minima on the PES (based on MP2/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) calculations), the cubic structure with D2d symmetry has been confirmed to be the global minimum. In a thermodynamic simulation using these 164 structures, the cubic structure with S4 symmetry has the highest population at low temperature, though double rings can become dominant as temperature going up, in good accord with a recent Monte Carlo simulation using an empirical potential. A transition temperature from cubic to noncubic has significantly been underestimated when potential energy data of B3LYP/6-311+G(d,p) calculations are employed in the simulation. This serious discrepancy between the MP2 and the B3LYP results suggests an importance of dispersion interactions for discussions on thermodynamics of water octamers.