Enhanced Septahedral Ordering in Cold Lennard-Jones Fluids

We report molecular dynamics calculations on two-component, cold (1.2 > T > 0.56 in natural units), three-dimensional Lennard-Jones fluids. Our system was small (7813 A, 7812 B particles), dense (N/V = 1.30), and distinctly finite (L \times L \times L cube, periodic boundary conditions, with L=22.96 \sigma_AA), \sigma_AA being the range of the $AA$ interaction in the Lennard-Jones potential U_{ij} = 4 \epsilon[(\sigma_ij/r)^12 -(\sigma_ij/r)^6]. 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 non-crystalline 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. Microviscosities inferred from mean-square particle displacements are exponential in static amplitude <(Q_7)^2>, and track closely in temperature dependence the orientation density slow mode lifetime. Volume wavelet decompositions show that when T is reduced, the correlation length of <(Q_7)^2> increases, especially below T=0.7, but the correlation length of <(Q_5)^2> is independent of T.