Persistence time for bonds in a tetravalent network fluid

The persistence of bonds for long times in a tetravalent saturated square-well model network fluid is studied from the gas to the glass transition by molecular dynamics simulation. The time correlation function for bonds fits a stretched exponential decay in time c(t) = exp (-(t/t p)β), which yields a persistence time t p. The time for c(t) to decay to zero, about 10t p, is a measure of the equilibration time, and corresponds to the time required for the root-mean-square displacement of the molecules to reach four molecular diameters. At liquid-like densities and for the temperature range covered, t p scales approximately as the inverse self-diffusion coefficient over four orders of magnitude. When the well depth ε/kT is large, t p, has an Arrhenius temperature dependence along isobars, a characteristic of strong liquids like SiO2, but when ε/kT = 0, the isobaric temperature dependence has the non-Arrhenius form of a more fragile liquid, like o-terphenyl.