Potential energy surfaces for water dynamics. II. Vibrational mode excitations, mixing, and relaxations

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Abstract
Dynamical behavior of liquid water is investigated by analyzing the potential energy surface involved. Multidimensional properties of the potential energy surface are explored in terms of vibrational mode excitations at its local energy minima, called inherent structures. The vibrational mode dynamics, especially mechanism of mode relaxation and structure transitions, is analyzed. It shows very strong excitation energy dependence and mode dependence. There are three kinds of vibrational coupling among modes. For excitations of energy near the room temperature, most modes (more than 90% of total modes) individually interact with only one or two other modes, and yield near recurrence of the mode energy in a few tens picoseconds (very slow relaxation). Spatially localized modes in the intermediate frequency range couple with many delocalized modes, yielding fast relaxation. The coupling is governed by atomic displacement overlaps and frequency matching. Each mode couples with nearby frequency or double frequency modes through the Fermi resonance. Lowest frequency modes almost always lead to transitions from a potential energy well to neighbor potential wells, called inherent structure transitions. In high energy excitation, some intermediate frequency modes also yield such transitions. There exist very low energy paths involving single or few water molecule displacements at almost every inherent structure, indicating that certain facile molecular movements occur even in very low temperature states. Different energy excitations of a low frequency mode result in different inherent structure transitions; transitions caused by high energy excitations involve many large molecular displacements. These inherent structure transitions are the source of the water binding structural reorganization dynamics. Significance of these vibrational mode dynamics in the water dynamics is discussed.

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