Viscosity and stress autocorrelation function in supercooled water: a molecular dynamics study

Following Guo, G.-J., and Zhang, Y.-G., 2001, Molec. Phys., 99, 283, which calculates the bulk and shear viscosities of SPC/E water at 30degreesC and 0.999 g cm(3), further molecular dynamics simulations have been performed at state points of 0degreesC, -20degreesC, -40degreesC, and -60degreesC along an approximate isobar with the previous state point. SACF and BACF (stress autocorrelation functions related to shear and bulk viscosities, respectively) of high precision have been obtained and compared for their similarities and differences. Shear and bulk viscosities calculated from them showed an increased deviation from real water with decreasing temperature. These correlation functions were then fitted using a uniform two-step relaxation function including a fast oscillatory Kohlrausch law and a slow straightforward Kohlrausch law. The fitting parameters of SACF and BACF have been analysed in detail, and several interesting dynamic phenomena were observed. (1) The oscillation frequency of SACF (44 similar to 48 ps(-1)) for short time intervals agrees with the stretching mode of hydrogen bonds, while that of BACF (7 similar to 12 ps(1)) agrees with the bending mode of hydrogen bonds. (2) With decreasing temperature, the slow relaxation fraction of the BACF increases, while that of the SACF remains constant. (3) The exponents beta in the Kohlrausch laws with values greater than 1 are obtained for BACF at ambient temperatures. (4) With regard to both shear and bulk viscosities, the slow relaxation time largely increases with decreasing temperature, while the fast relaxation time slightly decreases. These phenomena are qualitatively explained and discussed.