Molecular dynamics simulations of covalent amorphous insulators on parallel computers

Abstract

Algorithms are designed to implement molecular dynamics (MD) simulations on emerging concurrent architectures. A highly efficient multiresolution algorithm is designed to carry out large-scale MD simulations for systems with long-range Coulomb and three-body covalent interactions. Large-scale MD simulations of amorphous silica are carried out on systems containing up to 41 472 particles. The intermediate-range order represented by the first sharp diffraction peak (FSDP) in the neutron static structure factor shows a significant dependence on the system size. Correlations in the range 4–11 Å are found to play a vital role in determining the shape of the FSDP correctly. The calculated structure factor for the largest system is in excellent agreement with neutron diffraction experiments, including the height of the FSDP. Molecular dynamics simulations of porous silica, in the density range 2.2-0.1 g/cm 3, are carried out on a system of 41 472 particles. The internal surface area, pore surface-to-volume ratio, pore-size distribution, and other structural correlations are determined as a function of the density. Various dissimilar porous structures with different fractal dimensions are obtained by controlling the preparation schedule and temperature. Pore interface growth and the roughness of internally fractured surfaces in silica glasses are investigated by MD simulations of 1.12 million particles. During uniform dilation, the pores coalesce and grow in size. When the mass density is reduced to 1.4 g/cm 3, the pores grow catastrophically to cause fracture. The roughness exponent for fracture surfaces, α = 0.87 ± 0.02, supports experimental claims about the universality of α. The nature of the phonon density-of-states due to low-energy floppy modes in crystalline and glassy states of the high-temperature ceramic Si 3N 4 is investigated. Floppy modes appear continuously in the glass as the connectivity of the system is reduced. In the crystal, they appear suddenly at 30% volume expansion. The density-of-states due to the floppy modes varies linearly with energy, and the specific heat is significantly enhanced by the floppy modes.