STRUCTURE AND DYNAMICS OF SELENIUM CHAIN MELTS - A MOLECULAR-DYNAMICSSTUDY

A molecular dynamics (MD) study of liquid selenium modeled by 16 linea r chains of 40 monomers each is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kg m-3 at 873 K . The structural and force constant data of the chains were obtained f rom previous studies of neutron diffraction experiments, lattice dynam ics, and first principles calculations. The computed structural proper ties show a good agreement with available neutron scattering data. The flexibility of the chains and the high temperature thermodynamic stat e of the liquid enabled the observation of fast torsional motions and different spatiotemporal dynamic ranges, which can be described by the Rouse model for dense polymer solutions. We identify the crossover fr om an atomic to an intermediate or ''universal'' chain regime, and sub sequently to global chain behaviors. The dynamics of the system is dis cussed in terms of time and space-dependent transport coefficients. Th e generated MD trajectory thus provides information on the single part icle motions, the collective dynamics of one chain, and the dynamics o f the global system. This separation is useful for understanding the l ow frequency collective motions which can be measured by inelastic neu tron scattering. The spectra are interpreted in terms of existent dyna mical models, which imply a degree of trapping of the atoms in some sp atial regions of the liquid (''chain cages'') defined by atomic crossl inks, plus a slow diffusive process which modifies the shape of the ca ge according the renewal of the atomic crosslinks.