Molecular dynamics study of orientational cooperativity in water
Open Access
- 10 April 2006
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review E
- Vol. 73 (4), 041505
- https://doi.org/10.1103/physreve.73.041505
Abstract
Recent experiments on liquid water show collective dipole orientation fluctuations dramatically slower than expected (with relaxation time ) [D.P. Shelton, Phys. Rev. B 72, 020201(R) (2005)]. Molecular dynamics simulations of extended simple point charge (SPC/E) water show a large vortexlike structure of the dipole field at ambient conditions surviving over [J. Higo et al., Proc. Natl. Acad. Sci. U.S.A. 98, 5961 (2001)]. Both results disagree with previous results on water dipoles in similar conditions, for which autocorrelation times are a few picoseconds. Motivated by these recent results, we study the water dipole reorientation using molecular dynamics simulations of the SPC/E model in bulk water for temperatures ranging from ambient down to the deep supercooled region of the phase diagram at . First, we calculate the dipole autocorrelation function and find that our simulations are well described by a stretched exponential decay, from which we calculate the orientational autocorrelation time . Second, we define a second characteristic time, namely, the time required for the randomization of molecular dipole orientation, the self-dipole randomization time , which is an upper limit on ; we find that . Third, to check if there are correlated domains of dipoles in water which have large relaxation times compared to the individual dipoles, we calculate the randomization time of the site-dipole field, the net dipole moment formed by a set of molecules belonging to a box of edge . We find that the site-dipole randomization time for , i.e., it is shorter than the same quantity calculated for the self-dipole. Finally, we find that the orientational correlation length is short even at low .
Keywords
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