Application of the independent molecule model to the calculation of free energy and rigid-body motions of water heptamers and octamers

Stabilization of clusters of water heptamer and octamer in the gas phase is studied with the independent molecule model. As indicators of stabilization, the H-bonding strength, the H-bonding lifetime and the free energy are calculated. For both the heptamers and octamers, clusters with one oxygen-ring and some branching H-bonded waters are more stable than clusters with multi-oxygen-rings. Both the heptamer and octamer prefer a shape intermediate between planar and polyhedral; the branched waters are highly fluctuating and consequently contribute to increasing the entropy of the cluster. As the temperature increases from 0 to 300 K, the entropy contributes to the differentiation in free energy of the cluster geometries.