Potential energy surfaces of supercooled water: Intrabasin and interbasin structures explored by quenching, normal mode excitation, and basin hopping

Abstract

We investigate the potential energy surfaces of supercooled water, both intrabasin structures and distributions of the potential energy basins in configuration space by the steepest descent quenching and the normal mode excitation. The paths from liquid configurations to the corresponding local energy minima and the root mean square distance exhibit different characters below and above the temperature 213 K where the thermodynamics and structural properties change abruptly. The root mean square distance in the temperature range (298 K to 173 K) is larger than the corresponding harmonic oscillator system and it drastically increases above 223 K. The complexity also increases along the steepest descent path by increasing the number of inflection points. In order to investigate the other potential energy basins distributed in the vicinity of the (central) basin that the molecular dynamics trajectory visits, the distributions of these basins are examined by the normal mode excitation. The number of distinct basins increases with increasing temperature in the same volume of configuration space. The minimum potential energy of the adjacent basin at low temperature is almost always higher than the minimum potential energy of the central basin while that of the adjacent basin at high temperature is comparable to the central basin. The locations of the other basin centers are mostly orthogonal to the normal mode excitation. The potential energy surfaces are also examined by the basin hopping technique to seek for lower energy configurations started with a random and high energy molecular arrangement. It is found that energetically more stable molecules aggregate rather heterogeneously in the intermediate energy levels, which are hardly observed in the simulation, while the stable molecules distribute homogeneously in the lowest energy levels.