The potential energy surface, thermochemistry, and cooperative hydrogen bonding in the water octamer

The potential energy surface of the water octamer was explored through an extensive stochastic search, followed by MP2/6-311++G(d, p) and DLPNO-CCSD(T) refinement, yielding hundreds of structures grouped into seventy-seven distinct geometrical motifs. These structures were classified according to the topology of the hydrogen bond network, as determined from bond critical points in the quantum theory of atoms in molecules. The relative energies span a narrow window of less than 12 kcal per mole, and the thermal analysis reveals that several motifs become thermodynamically accessible at low temperatures and moderate pressures. The cooperative strengthening of the hydrogen bonds is rationalized through systematic shifts in the electron density, the energy density, and the virial ratio at the intermolecular critical points, all evaluated relative to the well characterized water dimer. Natural bond orbital analysis shows enhanced delocalization across the extended network, and the noncovalent interaction surfaces reveal the emergence of large connected regions of attraction in compact motifs. Together, these results establish a robust connection between network topology, cooperative electronic effects, and thermodynamic stability in the water octamer and provide a comprehensive view of the microscopic factors governing the structure and energetics of intermediate sized water clusters.