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Full-Text Articles in Physics
Force-Enhanced Atomic Refinement: Structural Modeling With Interatomic Forces In A Reverse Monte Carlo Approach Applied To Amorphous Si And Sio2, A. Pandey, Parthapratim Biswas, D. A. Drabold
Force-Enhanced Atomic Refinement: Structural Modeling With Interatomic Forces In A Reverse Monte Carlo Approach Applied To Amorphous Si And Sio2, A. Pandey, Parthapratim Biswas, D. A. Drabold
Faculty Publications
We introduce a structural modeling technique, called force-enhanced atomic refinement (FEAR). The technique incorporates interatomic forces in reverse Monte Carlo (RMC) simulations for structural refinement by fitting experimental diffraction data using the conventional RMC algorithm, and minimizes the total energy and forces from an interatomic potential. We illustrate the usefulness of the approach by studying a−SiO2 and a−Si. The structural and electronic properties of the FEAR models agree well with experimental neutron and x-ray diffraction data and the results obtained from previous molecular dynamics simulations of a−SiO2 and a−Si. We have shown that the method is more efficient …
Sculpting The Band Gap: A Computational Approach, Kiran Prasai, Parthapratim Biswas, D.A. Drabold
Sculpting The Band Gap: A Computational Approach, Kiran Prasai, Parthapratim Biswas, D.A. Drabold
Faculty Publications
Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations.
Relativistic Elastic Differential Cross Sections For Equal Mass Nuclei, Charles M. Werneth, Khin M. Maung, W.P. Ford
Relativistic Elastic Differential Cross Sections For Equal Mass Nuclei, Charles M. Werneth, Khin M. Maung, W.P. Ford
Faculty Publications
The effects of relativistic kinematics are studied for nuclear collisions of equal mass nuclei. It is found that the relativistic and non-relativistic elastic scattering amplitudes are nearly indistinguishable, and, hence, the relativistic and non-relativistic differential cross sections become indistinguishable. These results are explained by analyzing the Lippmann–Schwinger equation with the first order optical potential that was employed in the calculation.
A Generalized Force-Modified Potential Energy Surface For Mechanochemical Simulations, Gopinath Subramanian, Nithin Mathew, Jeff Leiding
A Generalized Force-Modified Potential Energy Surface For Mechanochemical Simulations, Gopinath Subramanian, Nithin Mathew, Jeff Leiding
Faculty Publications
We describe the modifications that a spatially varying external load produces on a Born-Oppenheimer potential energy surface (PES) by calculating static quantities of interest. The effects of the external loads are exemplified using electronic structure calculations (at the HF/6-31G- level) of two different molecules: ethane and hexahydro-1,3,5-trinitro-s-triazine (RDX). The calculated transition states and Hessian matrices of stationary points show that spatially varying external loads shift the stationary points and modify the curvature of the PES, thereby affecting the harmonic transition rates by altering both the energy barrier as well as the prefactor. The harmonic spectra of both molecules are blueshifted …
Structural Variation Of Alpha-Synuclein With Temperature By A Coarse-Grained Approach With Knowledge-Based Interactions, Peter Mirau, Barry L. Farmer, Ras B. Pandey
Structural Variation Of Alpha-Synuclein With Temperature By A Coarse-Grained Approach With Knowledge-Based Interactions, Peter Mirau, Barry L. Farmer, Ras B. Pandey
Faculty Publications
Despite enormous efforts, our understanding the structure and dynamics of α-synuclein (ASN), a disordered protein (that plays a key role in neurodegenerative disease) is far from complete. In order to better understand sequence-structure-property relationships in α-SYNUCLEIN we have developed a coarse-grained model using knowledge-based residue-residue interactions and used it to study the structure of free ASN as a function of temperature (T) with a large-scale Monte Carlo simulation. Snapshots of the simulation and contour contact maps show changes in structure formation due to self-assembly as a function of temperature. Variations in the residue mobility profiles reveal clear distinction among three …
Self-Assembly Dynamics For The Transition Of A Globular Aggregate To A Fibril Network Of Lysozyme Proteins Via A Coarse-Grained Monte Carlo Simulation, Ras B. Pandey, Barry L. Farmer, Bernard S. Gerstman
Self-Assembly Dynamics For The Transition Of A Globular Aggregate To A Fibril Network Of Lysozyme Proteins Via A Coarse-Grained Monte Carlo Simulation, Ras B. Pandey, Barry L. Farmer, Bernard S. Gerstman
Faculty Publications
The self-organizing dynamics of lysozymes (an amyloid protein with 148 residues) with different numbers of protein chains, Nc = 1,5,10, and 15(concentration 0.004 – 0.063) is studied by a coarse-grained Monte Carlo simulation with knowledge-based residue-residue interactions. The dynamics of an isolated lysozyme (Nc = 1) is ultra-slow (quasi-static) at low temperatures and becomes diffusive asymptotically on raising the temperature. In contrast, the presence of interacting proteins leads to concentration induced protein diffusion at low temperatures and concentration-tempering sub-diffusion at high temperatures. Variation of the radius of gyration of the protein with temperature shows a …
Impact Of Homogeneous Strain On Uranium Vacancy Diffusion In Uranium Dioxide, Anuj Goyal, Simon R. Phillpot, Gopinath Subramanian, David A. Andersson, Chris R. Stanek, Blas P. Uberuaga
Impact Of Homogeneous Strain On Uranium Vacancy Diffusion In Uranium Dioxide, Anuj Goyal, Simon R. Phillpot, Gopinath Subramanian, David A. Andersson, Chris R. Stanek, Blas P. Uberuaga
Faculty Publications
We present a detailed mechanism of, and the effect of homogeneous strains on, the migration of uranium vacancies in UO2. Vacancy migration pathways and barriers are identified using density functional theory and the effect of uniform strain fields are accounted for using the dipole tensor approach. We report complex migration pathways and noncubic symmetry associated with the uranium vacancy in UO2 and show that these complexities need to be carefully accounted for to predict the correct diffusion behavior of uranium vacancies. We show that under homogeneous strain fields, only the dipole tensor of the saddle with respect …
Lifetime Measurement Of The Cesium 6p3/2 Level Using Ultrafast Pump-Probe Laser Pulses, B. M. Patterson, J. F. Sell, T. Ehrenreich, M. A. Gearba, G. M. Brooke, J. Scoville, R. J. Knize
Lifetime Measurement Of The Cesium 6p3/2 Level Using Ultrafast Pump-Probe Laser Pulses, B. M. Patterson, J. F. Sell, T. Ehrenreich, M. A. Gearba, G. M. Brooke, J. Scoville, R. J. Knize
Faculty Publications
Using the inherent timing stability of pulses from a mode-locked laser, we measure the cesium 6P3/2 excited-state lifetime. An initial pump pulse excites cesium atoms in two counterpropagating atomic beams to the 6P3/2 level. A subsequent synchronized probe pulse ionizes atoms that remain in the excited state and the photoions are collected and counted. By selecting pump pulses that vary in time with respect to the probe pulses, we obtain a sampling of the excited-state population in time, resulting in a lifetime value of 30.462(46) ns. The measurement uncertainty (0.15%) is slightly larger than our previous report of …