Electric-Field Induced Structural Evolution of Water Clusters (H$_{2}$O)$_{n}$ [n = 9 - 20]: \emph {Ab initio} Density Functional Approach

Response of neutral water clusters (H$_{2}$O)$_{n}$, n = 9 through 20; to external electrostatic fields (0 to 1V/{\AA}), is studied for the lowest-energy conformers within in the energy band $\sim$2 kcal/mol, for each ``n,'' employing the versatile B3LYP hybrid prescription of density functional theory in conjunction with the polarized basis 6-311++G(2d, 2p). Increase in the field elongates and weakens hydrogen-bond networks; ``opening up'' three-dimensional cluster morphologies to complex net-like structures culminating into their disintegration at specific threshold values. All conformations are stable: they manifest as local minima on their potential energy surfaces. Field-induced structural transitions are invariably accompanied by an abrupt increase in the electric dipole moment, which is marked at breakdown, where the highest-occupied and lowest-unoccupied molecular orbital energy gap diminishes to zero. Remarkably, as a consequence of their zero or very small electric dipole moments, certain conformers endowed with molecular symmetry exceptionally exhibit a peculiar behavior: they either remain completely robust to increase in the field, or break up into smaller, identical, robust building units with tetramer, pentamer and hexamer ring- or cubic-geometries.