ATOMIC SELF-MOTION IN LIQUID LEAD - MODE-COUPLING ANALYSIS AND EVIDENCE FOR STRUCTURAL EFFECTS IN SELF-DIFFUSION

Atomic self-motion in liquid lead at 623 K and 1170 K was studied by MD simulation using a system of 16 384 particles. The simulation was b-ed on the effective pair potential derived earlier from the measured structure factor. The incoherent intermediate scattering function F(s)(Q, t) derived in the Q-range between 0.077 and 7.7 angstrom-1, and its deviation from the Langevin approximation was analyzed using a convenient double-scaling presentation. The most interesting result of this analysis is that at T = 623 K the magnitude of this non-Langevin component was found to display a characteristic pattern of Q-dependence, which resembles that of the static structure factor S(Q). Some elements of this picture of Q-dependence were also found in the high-temperature region (K.-E. Larsson et al., Phys. Rev. A 46 (1992) 1132). It is not clear if the mode-coupling theory can account for these structure effects. Mode-coupling analysis of the same data (W. Gudowski et al., Phys. Rev. E 47 (1993) 1693) shows the significant discrepancies between theoretical predictions and MD results. Furthermore, the second-order memory function of the density self-correlation function which is used to calculate the coupling terms in the mode-coupling formalism, decays to zero-value in about 2.5 ps, too early to be immediately identified with the structure-coupled effects described here.