Showing papers by "Paul C. Whitford published in 2005"

Enhanced septahedral ordering in cold Lennard-Jones fluids.

Abstract: We report molecular dynamics calculations on a two-component, cold, 15 625 particle, three-dimensional Lennard-Jones fluid. We calculated spherical harmonic components ${Q}_{LM}$ for the density of particles in the first coordination shell of each particle, as well as their spherical invariants $⟨{Q}_{L}^{2}⟩$, time-correlation functions, and wavelet density decompositions. The spherical invariants show that noncrystalline septahedral $(⟨{Q}_{7}^{2}⟩)$ ordering is important, especially at low temperature. While $⟨{Q}_{10}^{2}⟩$ could arise from icosahedral ordering, its behavior so closely tracks that of the nonicosahedral $⟨{Q}_{11}^{2}⟩$ that alternative origins for $⟨{Q}_{10}^{2}⟩$ need to be considered. Time correlation functions of spherical harmonic components are bimodal, with a faster temperature-independent mode and a slow, strongly temperature-dependent mode. Volume wavelet decompositions show that when $T$ is reduced, the correlation length of $⟨{Q}_{7}^{2}⟩$ increases, especially below $T=0.7$, but that the correlation length of $⟨{Q}_{5}^{2}⟩$ is independent of $T$.

Extended-range order, diverging static length scales, and local structure formation in cold Lennard-Jones fluids

Abstract: We report molecular-dynamics simulations on a three-dimensional, two-component Lennard-Jones fluid. We used 125 000 particles (equal numbers of A and B) at density N/V=1.29 and 34 temperatures T covering 5×104⩾T⩾0.56. The pair potential was 4e[(σij/r)12−(σij/r)6] with σAA=1, σAB=11/12, and σBB=5/6. We computed specific and generic radial distribution functions gij(r), and several density-momentum dynamic correlation functions whose static (t=0) parts vanish by symmetry. Evidence is presented that our systems were adequately annealed to eliminate remnant initial order and were adequately equilibrated at each temperature. Static spatial correlations in cold Lennard-Jones liquids have longer ranges than are often reported: g(r)−1≠0 is found out to r⩾7 at T=2 and out to r⩾10 at T=0.56. |g(r)−1| has an envelope function that simultaneously fits both crests and troughs of g(r). The envelope function implies a temperature-dependent static length scale l1; over (0.56⩽T⩽100), l1∼T−0.3, contrary to suggestions that...