Structural and energetic stability of the lowest equilibrium structures of water clusters

Water molecules with their hydrogen bonding capability, exhibit exceptional properties in the bulk as well as in cluster form. In the present work, we study the size-dependent trends in the structure, energetics, bonding, ionisation potential, fragmentation pattern, and optical properties of water clusters in the size range of n = 2–20, and an interplay between them. We have extensively searched for the lowest energy structures of the water clusters using the artificial bee colony algorithm, optimised them first with the classical force field TIP4P and then relaxed at least 10 lowest energy structures using density functional theory. We have found new lowest energy structures for all the sizes as against the ones reported earlier. The structures and stability of water clusters are primarily dictated by the H-bond network. However, we found the weak van der Waals interactions also play a crucial role in stabilising the clusters giving them unique characteristics. Some of the clusters such as those with $$n=4, 8, 10, 12$$ n = 4 , 8 , 10 , 12 and 15 molecules were structurally symmetric, yet a close analysis of various properties reveals that the clusters with $$n=4, 8, 12, 14$$ n = 4 , 8 , 12 , 14 and 19 molecules are more stable than others. Spherical or nearly spherical clusters were found to be the most stable, corroborated by the shape deformation parameters and the fragmentation pattern, which indicated a higher likelihood of forming fragments of sizes $$n=4, 8, 12, 14$$ n = 4 , 8 , 12 , 14 , and 16. A blueshift of the H-O-H vibrational modes and a redshift of the O–H stretching modes is seen for most clusters. Such characteristics in the vibrational spectra is associated with an increase in the H-bond strength which is seen to increase with size of the cluster. Large optical band gaps for $$n=4, 8, 12$$ n = 4 , 8 , 12 and 16 along with blueshifts in optical spectra implies these clusters to be chemically more stable than others.