Energy landscape view of hydrogen-bond rearrangement dynamics

1 Energy landscape view of hydrogen-bond rearrangement dynamics 

To identify the intermittent collective molecular motions associated with the HB network rearrangement in water, IS analysis is employed, as discussed in Chapter 4. Inherent structure analysis is used to identify the local minima (called the ISs, which are considered as quasi-stable stmctures) of the potential-energy wells, which are sequentially visited by the system along the trajectory. In the trajectory, the system undergoes well-to-well transitions (IS transitions) after exhibiting vibrational motions in individual wells. The sum is over all the water molecules. 

Such IS analysis has proved to be quite useful in providing a semi-quantitative description of water dynamics. From these ISs, one can construct a time-distance matrix, defined by [5] 

We can obtain certain sets of sequential ISs from island stmctures (basins) they are mutually separated with small energy barriers and small stmcture differences. The transitions between ISs within a basin (minor IS transitions) occur easily and frequently [5]. 

Such an analysis shows that among the ISs visited by the system, there are islands (basins) of various sizes, distributed randomly in time, and thus the transitions among the islands (called the basin-basin transitions) occur intermittently. The ISs of water are different from other liquids. As the temperature is lowered, the system spends longer and longer times in a few basins. The basin-basin transitions comprise large IS changes but may often involve relatively small energy barriers [5]. 

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