On the short time motion of hydrogen-bonded molecules in supercooled water

The short time dynamics of tagged pairs of molecules that, at the initial t ime, are in the first coordination shell, is investigated in supercooled li quid water at 245 K by using the molecular dynamics technique with the four -points transferable intermolecular potential of Jorgensen [J. Chem. Phys. 79, 926 (1983)]. The instantaneous normal mode approach and the results of the local structure investigations are exploited to build up a correlation function of the relative displacements that represents the projection of no rmal modes along the initial center of mass separation vector. By imposing simple constraints to the initial dynamical conditions, localized damped os cillations of the centers of mass are detected along the hydrogen bond dire ctions. The corresponding density of states shows a maximum around the freq uency of 230 cm(-1) and its shape agrees with the frequency contributions e xpected from the translational phonon branches of ice. Total and radial cor relation functions of the relative velocity are also computed. The radial c omponent is dominated by oscillations at the frequency of 230 cm(-1); it co mpares fairly well with that derived from the translational density of stat es, thus supporting the reliability of the employed method. A slower compon ent of motion, absent along the hydrogen bond direction, is derived; in the density of states, it produces a contribution peaked around 50 cm(-1). (C) 1999 American Institute of Physics. [S0021-9606(99)51734-0].