Low temperature heat capacity of water clusters

Geometry optimization and vibrational frequency calculation are carried out at the MP2/6-31G(d,p) level for 35 low-energy isomers of (H2O)n clusters in the size range n = 6–21. The heat capacities of the clusters are calculated using quantum statistical theories based on the harmonic approximation. The specific heat capacity increases with the cluster size but the difference diminishes gradually with increasing size. The heat capacities divided by the number of intermolecular vibrational modes are very close for all the clusters. The overall picture of the heat capacity of the clusters is bulk-like and it agrees well with the experimental results of size-selected clusters.