Molecular dynamics study of orientational cooperativity in water.

Recent experiments on liquid water show collective dipole orientation fluctuations dramatically slower than expected (with relaxation time > 50 ns) [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, 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 300 K down to the deep supercooled region of the phase diagram at 210 K. 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 t(alpha). Second, we define a second characteristic time, namely, the time required for the randomization of molecular dipole orientation, the self-dipole randomization time t(r), which is an upper limit on t(alpha); we find that t(r) is approximately equal to 5t(alpha). 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 t(box) of the site-dipole field, the net dipole moment formed by a set of molecules belonging to a box of edge L(box). We find that the site-dipole randomization time t(box) is approximately equal to 2.5t(alpha) for L(box) approximately equal to 3 A, 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 T.