Classical and quantum simulations for large systems on parallel computers

Molecular dynamics (MD) method is used to investigate structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures and the modification of SiO2 glass network at very large negative pressures. The nature of molecular vibrations in solid C60 has been studied with tight binding molecular dynamics (TBMD) method. Implementations of simulation algorithms on parallel computers are also discussed. In a-SiO2 at high pressures, the height of the first sharp diffraction peak in S(q) is considerably diminished and its position shifts to larger wave vectors. At twice the normal density, Si-O bond length increases, Si-O coordination changes from 4 to 6, and O-Si-O band-angle changes from 109° to 90°. This is clearly a tetrahedral to octahedral transformation, which is observed recently by Meade, Hemley, and Mao in their diffraction experiments using synchrotron radiation. MD simulations of porous silica are carried out in the density range 2.2-0.1 g/cm3. Internal surface area, pore surface-to-volume ratio, gyration radius, and fractal dimension are studied as a function of density. Simulations are in good agreement with the experimental results obtained by x-ray scattering. The results reveal a crossover of the structural correlations between short- to intermediate-range (<8 angstroms) and fractal- to large-scale-regime (10 to approximately 100 angstroms). Dispersion and density of states (DOS) of inter- and intra-molecular phonons are calculated for orientationally ordered and disordered solid C60 using the TBMD method. Inter-molecular phonon DOS extends up to 7.6 meV and shows libron peaks at 2.4 meV and 3.7 meV in the orientationally ordered phase. Orientational disorder softens libron modes. Intra-molecular phonons below 70 meV also show significant dispersion. Our results are in good agreement with the recent inelastic neutron scattering experiments. MD is a numerical approach which involves the solution of Newton's equations for particles in the system. The multiple-time-step (MTS) approach reduces this computation significantly by exploiting the different time scales for short-range and intermediate-range interactions. Using the linked-list scheme, parallel algorithms are designed to implement on the in-house 8-node iPSC/860, a MIMD (multiple instruction multiple data) machine. Our group has also developed a quantum dynamical simulation scheme for Computational Nanoelectronics based on the quantum molecular dynamics (QMD) method. The QMD algorithm has been implemented on the in-house 8,192-node MasPar, a SIMD (single instruction multiple data) architecture.