Showing papers by "Bobby G. Sumpter published in 1993"

Molecular dynamics simulations of the hexagonal structure of crystals with long methylene sequences

Abstract: An unconstrained polymethylene crystal, consisting of 9600 CH 2 groups, in which each CH 2 is permitted to carry out stretching, bending, and torsional motion, has been studied at various temperatures using molecular dynamics simulations. Information about the atomistic details of the dynamics and structure of these crystals is presented. A significant disorder exists at temperatures well below the melting point. Close to melting, the disordered crystals have about 2% of gauche bonds that are distributed mainly at positions close to the surface of the crystal. The major disorder consists, however, of a collective twisting of the chains leading to a hexagonal crystal structure. The hexagonal structure of the symmetric motifs is caused by a dynamic multidomain arrangement of the chains

Atomistic details of disordering processes in superheated polymethylene crystals

Abstract: Atomistic details of disordering in superheated polymethylene crystals have been studied using molecular dynamics simulations. Superheating causes a quick development of large-scale disorder throughout the crystal, including reorientation, translation, and the destruction of crystal symmetry, followed ultimately by surface melting. Crystallization-centers with hexagonal packing are found in the superheated, unconstrained crystals. On cooling during the simulation, a recrystallization process competes with the disordering resulting in a reorientation of the molecular chains and reorganization of the crystal.

Dynamics of a paraffin crystal

Abstract: Recently it has become possible to make a series of simulations on a supercomputer of the molecular motion of a pentacontane (C50H102) crystal containing close to 10000 atoms. Such a crystal is big enough to serve as a model of a macroscopic crystal. It can also serve as a model for polyethylene. The large volume of detailed information on the structure and motion permits a new level of understanding. Information on crystal appearance, pseudo-symmetric crystal structures, picosecond time-scale formation of conformational defects and the beginning of the melting process is presented.

Neural Network Simulations on Massively Parallel Computers: Applications in Chemical Physics

Abstract: A fully connected feedforward neural network is simulated on a number of parallel computers (MasPar-1, Connection Machine CM5, Intel iPSC-2 and iPSC-860) and the performance is compared to that obtained on sequential vector computers (Cray YMP, Cray C90, IBM-3090) and to a scaler workstation (IBM RISC-6000). Peak performances of up to 342 million connections per second (MCPS) could be obtained on the Cray C90 using a single processor while the optimum performance obtained on the parallel computers was 90 MCPS using 4096 processors. Efficiency such as these has enabled neural network computations to be carried out for a number of chemical physics problems. Several examples are discussed: multi-dimensional function/surface fitting, coordinate transformations, and predictions of physical properties from chemical structure.

Computer simulations of atomic force microscopy : crystalline polymers and the effects of surface contaminants

Abstract: The atomistic dynamics of the interaction of an atomic force microscopic (AFM) probe with a crystalline polyethylene surface was examined by using the molecular dynamics method. The results show that the internal dynamics of the polymer crystal is such that rapid relaxation occurs, providing for a large amount of structural reversibility and making it possible to perform nondestructive AFM experiments. However, surface and/or AFM tip defects or contaminants (such as those which can be induced by polar molecules adsorbed on the surface), can result in significant perturbations in the AFM images produced, causing large and sharp structures to appear on the surface topology. A rationale of the mechanisms responsible for the image distortions is presented, and a relationship to defects observed in AFM and STM experiments is given.

Atomistic Details of Disordering Processes in Superheated Polymethylene Crystals II. Effects of Surface Constraints

Abstract: Atomistic details of disordering in superheated polymethylene crystals have been studied using full molecular dynamics simulations of crystals containing 9600 CH 2 -groups. The crystal size was about 227 nm 3 Simulations were carried out for up to 100 ps, starting at temperatures about 100 K above the melting temperature. Typically 1.5 h of CPU time on a Cray X-MP were necessary per ps simulation. Superheating causes a quick development of large-scale disorder throughout the crystal, including reorientation, translation, and the destruction of crystal symmetry. This is followed ultimately by surface Melting. Crystallization centers with hexagonal packing are found in superheated, unconstrained crystals. On cooling during the simulation, recrystallization processes compete with the disordering, resulting in a reorientation of the molecular chains and reorganization of the crystal. Neither the fully amorphous phase nor the ordered crystal are reached during these short-time simulations using an instantaneous temperature increase to above the melting temperature, followed by a slow cooling into the crystallization temperature region.