STUDY OF THE ALPHA AND BETA RELAXATIONS IN A SUPERCOOLED FLUID VIA MOLECULAR-DYNAMICS SIMULATIONS

Incoherent scattering function (self-part of the density autocorrelati on function) F(s)(k, t) is computed by molecular-dynamics (MD) simulat ion in supercooled fluids of binary soft-sphere mixtures. The full den sity autocorrelation function F(k, t) was also computed. It is found t hat F(s)(k, t)'s at various temperatures and wavenumbers can be fitted over a wide range of time steps (at least over three orders of the de cay) by a Williams-Watts stretched exponential function F(s)(k, t) = A exp[-(t/t0)beta], where A, beta and t0 are adjustable parameters. Sig nificant dynamical behaviours are also presented for mean square displ acements and non-Gaussian parameters. With results obtained from diffe rent system size, N = 500 and N = 4000 a significant size dependence i s suggested. Generalized susceptibility chi(k, omega) and dynamical st ructure factor S(k, omega) are also computed over a wide range of omeg a (over five orders) using a new algorithm for the numerical integrati ons. Computational results are presented for the imaginary part of the generalized susceptibility, chi''(k, omega), which indicates both alp ha and beta peaks in such spectra for the first time by the present MD computation. The present MD results are in good agreement with the pr edictions of the trapping diffusion model, which we have previously pr oposed for the glass transition.