Richard Ge - Academia.edu (original) (raw)

Papers by Richard Ge

ACM SIGCSE Bulletin, 1991

Polymer, 2004

This is a preprint of a paper intended for publication in a journal or proceedings. Since changes... more This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.

The Journal of Physical Chemistry B, 2006

Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many indu... more Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many industrial processes. We use Density Functional Theory (DFT) to investigate the chemistry of one such system, i.e., 1,4diphenyl-butadiyne, or DPB, in contact with Pd (110) and (111) surfaces. Reaction pathways and energetics of important processes are explored, including H 2 adsorption, dissociation and migration on the metal surface, DPBmetal interaction, the energetics of H uptake, and the effects of impurities like CO and CO 2 on H chemistry. We find that: (i) strong aromatic-metal interaction leads to significant binding strength of DPB molecule to both Pd surfaces, especially the (110); (ii) H 2 molecule readily dissociates on the Pd surface into H-radicals, which get taken up by alkyne triple bonds; (iii) CO has strong binding to the metal surface, but interacts weakly with H radicals; (iv) CO 2 binds weakly to the metal surface, but could potentially lead to interesting chemical reactions with H.

Skeletal Radiology, 1992

Background: Cystic nephroma It is a benign lesion that can mimic a malignant tumor of the kidney.... more Background: Cystic nephroma It is a benign lesion that can mimic a malignant tumor of the kidney. Case presentation: Here we report a case of multilocular cystic nephroma with unusual localization, largely invading the pyelocaliceal cavities. Conclusions: The importance of its accurate diagnosis is to avoid a radical surgery proposing a partial surgery to preserve a vital organ.

Polymer, 1995

Molecular dynamics (MD) simulations have been used to study the diffusion of methane as a small m... more Molecular dynamics (MD) simulations have been used to study the diffusion of methane as a small molecule penetrant example in cis-1, 4-polybutadiene (PBD). The non-bonded potential for an 'anisotropic'united atom (AUA) representation of the CH= group was ...

Physical Review Letters, 2006

We predict that coherent electromagnetic radiation in the 1-100 THz frequency range can be genera... more We predict that coherent electromagnetic radiation in the 1-100 THz frequency range can be generated in crystalline materials when subject to a shock wave or solitonlike propagating excitation. To our knowledge, this phenomenon represents a fundamentally new form of coherent optical radiation source that is distinct from lasers and free-electron lasers. The radiation is generated by the synchronized motion of large numbers of atoms when a shock wave propagates through a crystal. General analytical theory and NaCl molecular dynamics simulations demonstrate coherence lengths on the order of mm (around 20 THz) and potentially greater. The emission frequencies are determined by the shock speed and the lattice constants of the crystal and can potentially be used to determine atomic-scale properties of the shocked material.

Physical Review B, 2007

We have previously predicted that coherent electromagnetic radiation in the 1-100 THz frequency r... more We have previously predicted that coherent electromagnetic radiation in the 1-100 THz frequency range can be generated in crystalline polarizable materials when subject to a shock wave or solitonlike propagating excitation ͓E. J. Reed et al., Phys. Rev. Lett. 96, 013904 ͑2006͔͒. In this work, we present analysis and molecular dynamics simulations of shock waves in crystalline NaCl which expand upon this prediction. We demonstrate that the coherent polarization currents responsible for the effect are generated by a nonresonant, nonlinear effect that occurs at the shock front. We consider the effect of thermal noise and various shock pressures on the coherent polarization currents and find that the amplitude generally increases with increasing shock pressure and decreasing material temperature. Finally, we present calculations of the amplitude and distribution of emitted radiation showing that the radiation can be directed or undirected under various realistic conditions of the shape of the shock front.

Nature Materials, 2009

In the above Letter, the parameters for the nonpolar polymer model used in the simulations were i... more In the above Letter, the parameters for the nonpolar polymer model used in the simulations were incorrectly specified as being identical to the polyethylene model of Paul et al. (ref. 21 of the Letter). However, the potential used in the Letter differed from that of Paul et al. in that the twofold term in the CC -CC torsion was changed from k 2 Φ (1-cos2Φ), used in the polyethylene (PE) polymer model of Paul et al. to k 2 Φ (1+cos2Φ).

Macromolecules, 2011

ABSTRACT The dynamical characteristics of ring and linear polyethylene (PE) molecules in the melt... more ABSTRACT The dynamical characteristics of ring and linear polyethylene (PE) molecules in the melt have been studied by employing atomistic molecular dynamics simulations for linear PEs with carbon atom numbers N up to 500 and rings with N up to 1500. The single-chain dynamic structure factors S(q,t) from entangled linear PE melt chains, which show strong deviations from the Rouse predictions, exhibit quantitative agreement with experimental results. Ring PE melt chains also show a transition from the Rouse-type to entangled dynamics, as indicated by the characteristics of S(q,1) and mean-square monomer displacements g(1)(t). For entangled ring PE melts, we observe g(1)(t) similar to t(0.35) and the chain-length dependence of diffusion coefficients D(N) proportional to N(-1.9), very similar to entangled linear chains. Moreover, the diffusion coefficients D(N) remain larger for the entangled rings than the corresponding entangled linear chains, due to about a 3-fold larger chain length for entanglement. Since rings reptate, our results point toward other important dynamical modes, based on mutual relaxations of neighboring chains, for entangled polymers in general. do not

Macromolecules, 1994

Progress has been made recently in using molecular dynamics (MD) simulations to generate PVT prop... more Progress has been made recently in using molecular dynamics (MD) simulations to generate PVT properties of amorphous polymers. In the present work previous MD simulations of V-T curves for several polymers are extended to lower temperatures, including through the glass transition. It is demonstrated that these V-T curves can be used to locate volumetric glass transition temperatures (T,) reliably. Four polymers, namely, cis-poly(1,3-butadiene), polyisobutylene, atactic polypropylene, and polystyrene were studied, and previously determined MD data for polyethylene (PE) are available. The Tg values span a range of 200 K, from 170 to 370 K. The values from .the MD V-T curves tend to be displaced, as expected, to somewhat higher temperature than the longer time experimental values. However, the displacements are minor compared to the range of Tg values considered. Determination of Tg from MD simulations appears to be a practical procedure. The relation of the MD-determined Tg of wholly amorphous PE to experimental values in the semicrystalline environment is discussed.

Macromolecules, 2001

ABSTRACT We perform molecular dynamics simulations of chlorotrifluoroethylene (CTFE)/vinylidene f... more ABSTRACT We perform molecular dynamics simulations of chlorotrifluoroethylene (CTFE)/vinylidene fluoride (VDF) copolymers over a wide range of compositions and temperatures including the volumetric glass transition. Structural features of the polymers and copolymers were characterized using pair correlation functions and wide-angle X-ray scattering (WAXS) spectra. Calculated WAXS spectra for the CTFE/VDF copolymer known as Kel-F 800 are compared with experimental results and are found to compare well. The X-ray peak associated with interchain spacing is found to shift to larger scattering angles (smaller d spacings) as the concentration of the CTFE comonomer decreases, suggesting a transition between CTFE and VDF homopolymer interchain packing structures, consistent with earlier experiments. The strong compositional dependence is traced to packing involving the CF2 moiety. The calculated WAXS were further deconvoluted into atom−atom contributions in an attempt to explain the origin of the peak positions found in the WAXS. The location of the first peak in the spectra did not correspond to interchain packing distances for any of the polymer compositions except the VDF homopolymer. The effect of Coulombic interactions on polymer structure is examined. We find that the importance of Coulombic interactions is greatest for pVDF. The torsional angle distribution and the characteristic ratio show greater sensitivities to Coulombic interactions than the density and the total radial distribution function.

Journal of Crystal Growth, 2006

Starting from an accurate inter-atomic potential we develop a simple scheme of generating an "on-... more Starting from an accurate inter-atomic potential we develop a simple scheme of generating an "on-lattice" molecular potential of short range, which is then incorporated into a lattice Monte Carlo code for simulating size and shape evolution of nanocrystallites. As a specific example, we test such a procedure on the morphological evolution of a molecular crystal of interest to us, e.g., Pentaerythritol Tetranitrate, or PETN, and obtain realistic facetted structures in excellent agreement with experimental morphologies. We investigate several interesting effects including, the evolution of the initial shape of a "seed" to an equilibrium configuration, and the variation of growth morphology as a function of the rate of particle addition relative to diffusion.

The Journal of Chemical Physics, 1994

Molecular dynamics (MD) simulation has been used to study chain dynamics in bulk polyethylene ove... more Molecular dynamics (MD) simulation has been used to study chain dynamics in bulk polyethylene over a wide range of temperature. This range includes the volumetric glass transition. The latter was determined from the volume temperature relation established via constant pressure ...

The Journal of Chemical Physics, 2004

An all-atom force field for 1,3,5-triamino-2,4,6-trinitrobenzene ͑TATB͒ is presented. The classic... more An all-atom force field for 1,3,5-triamino-2,4,6-trinitrobenzene ͑TATB͒ is presented. The classical intermolecular interaction potential for TATB is based on single-point energies determined from high-level ab initio calculations of TATB dimers. The newly developed potential function is used to examine bulk crystalline TATB via molecular dynamics simulations. The isobaric thermal expansion and isothermal compression under hydrostatic pressures obtained from the molecular dynamics simulations are in good agreement with experiment. The calculated volume-temperature expansion is almost one dimensional along the c crystallographic axis, whereas under compression, all three unit cell axes participate, albeit unequally.

The Journal of Chemical Physics, 2003

Primary nucleation of poly(vinylidene fluoride) (pVDF) from bulk entangled amorphous polymer melt... more Primary nucleation of poly(vinylidene fluoride) (pVDF) from bulk entangled amorphous polymer melts has been observed from molecular dynamics (MD) simulations. This is the first instance of spontaneous primary nucleation of an entangled bulk polymer in a MD simulation. The melt-to-crystal phase transition occurs via a three-stage process. The ordered phase is found to consist of mainly chain-folded lamella with predominantly

Journal of child neurology, 2012

Recent progress in scientific research has facilitated accurate genetic and neuropathological dia... more Recent progress in scientific research has facilitated accurate genetic and neuropathological diagnosis of congenital myopathies. However, given their relatively low incidence, congenital myopathies remain unfamiliar to the majority of care providers, and the levels of patient care are extremely variable. This consensus statement aims to provide care guidelines for congenital myopathies. The International Standard of Care Committee for Congenital Myopathies worked through frequent e-mail correspondences, periodic conference calls, 2 rounds of online surveys, and a 3-day workshop to achieve a consensus for diagnostic and clinical care recommendations. The committee includes 59 members from 10 medical disciplines. They are organized into 5 working groups: genetics/diagnosis, neurology, pulmonology, gastroenterology/nutrition/speech/oral care, and orthopedics/rehabilitation. In each care area the authors summarize the committee's recommendations for symptom assessments and therape...

Chemical Physics Letters, 2003

Electronic structure calculations of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) clusters are pre... more Electronic structure calculations of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) clusters are presented. The calculated gas phase structures of TATB are close to the experimental crystal structures. Two isomers of the TATB dimer are considered. One resembles the structures found experimentally for the A-B sheet of the molecular crystal. The other, a stacked ring (ÔCÕ) configuration, yields an eclipsed structure, while the crystal data suggest two rings displaced relative to each other. Electron correlation effects are of greater importance for the stacked C-dimer than the planar AB-dimer. Furthermore, many-body contributions to the total interaction energy are found to be of limited importance.

Applied Physics Letters, 2007

Careful thermometric analysis (TMA) on powders of micron-sized triamino-trinitrobenzene (TATB) cr... more Careful thermometric analysis (TMA) on powders of micron-sized triamino-trinitrobenzene (TATB) crystallites are shown to display irreversible growth in volume when subjected to repeated cycles of heating and cooling. Such behavior is counter-intuitive to typical materials response to simulated annealing cycles in atomic-scale molecular dynamics. However, through coarse-grained simulations using a mesoscale Hamiltonian we quantitatively reproduce irreversible growth behavior in such powdered material. We demonstrate that irreversible growth happens only in the presence of intrinsic crystalline anisotropy, and is mediated by particles much smaller than the average crystallite size.

ACS Nano, 2009

Molecular dynamics simulations are used to study the influence of functionalized substrates on th... more Molecular dynamics simulations are used to study the influence of functionalized substrates on the orientation of poly(3-hexylthiophene) (P3HT) nanocrystallites, which in turn plays a critical role in P3HT-based transistor performance. The effects of alkyl-trichlorosilane self-assembled monolayer packing density, packing order, and end-group functionality are independently investigated. Across these factors, the potential energy surface presented by the substrate to the P3HT molecules is determined to be the main driver of P3HT ordering. Surprisingly, disordered substrates with a smoothly varying potential energy landscape are found to encourage edge-on P3HT orientation, while highly ordered substrates have undesirable potential energy wells that reduce the edge-on orientation of P3HT because of substrate؊side-chain interactions.