Metabasin dynamics and local structure in supercooled water

Abstract

We employ the Distance Matrix method to investigate metabasin dynamics in supercooled water. We find that the motion of the system consists in the exploration of a finite region of configuration space (enclosing several distinct local minima), named metabasin, followed by a sharp crossing to a different metabasin. The characteristic time between metabasin transitions is comparable to the structural relaxation time, suggesting that these transitions are relevant for the long time dynamics. The crossing between metabasins is accompanied by very rapid diffusional jumps of several groups of dynamically correlated particles. These particles form relatively compact clusters and act as cooperative relaxing units responsible for the density relaxation. We find that these mobile particles are often characterized by an average coordination larger than four, i.e. are located in regions where the tetrahedral hydrogen bond network is distorted.

Statistical identification of structural rearrangement events in molecular dynamics trajectories

“…Examples for those sudden changes in single molecule experiments are enzymatic turnovers [1], motor protein movements [2], nanoparticle and single molecule blinking [3][4][5][6][7][8], and single molecule reorientations in a glassy matrix [9]. Likewise, in molecular dynamics simulations sudden jumps have been observed in the position of the simulated atoms, for example in binary Lennard-Jones systems [10][11][12], protein folding simulations [13], and supercooled water [14]. These sudden rearrangements have even been observed experimentally in colloidal model systems of glassy dynamics [15].…”

Temperature dependence of the structure of protein hydration water and the liquid-liquid transition

“…A high value of I (i,t) implies that molecule i at time t has a good tetrahedral local order and low local density [and thus, a low local potential energy since it is able bind to its first four neighbors by geometrically well-shaped hydrogen bonds (HB)], while on the contrary, values of I (i,t) ≈ 0 indicate a molecule with defective tetrahedral order and high-local density (and thus, high local potential energy), even allowing for a fifth neighbor within the coordination shell. This abnormal coordination could also promote the formation of bifurcated hydrogen bonds (when a water molecule binds to two others via the same hydrogen), a feature that has been shown to promote local mobility [40,41]. Liquid water both in the normal liquid state and in the supercooled liquid state has been shown to present I distributions with a peak at low values and a tail to the right.…”

“…[9]. In order to find HBs we used a geometric criterion (which in past studies [9,41,42,44] has been shown to provide reasonable results also consistent with more elaborated criteria): A HB implies an O-O distance of less than 3.5Å and an angle O-H· · ·O greater than 140 • (optimum angle for HB would be of 180 • ). In Fig.…”

Spatial arrangements of particles with different mobility tendencies in a model glass-former

“…These relatively compact clusters (named ''democratic'' clusters or d-clusters [16]) have been identified in different glassy systems like a binary Lennard-Jones system and supercooled water [19] and have recently received experimental support from a mixed experimental-computational work in a glassy polymer [20].…”