Khorgolkhuu Odbadrakh

Effect of charge distribution on RDX adsorption in IRMOF-10.

Quantum mechanical (QM) calculations, classical grand canonical Monte Carlo (GCMC) simulations, and classical molecular dynamics (MD) simulations are performed to test the effect of charge distribution on hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) adsorption and diffusion in IRMOF-10. Several different methods for mapping QM electron distributions onto atomic point charges are explored, including the electrostatic potential (ESP) method, Mulliken population analysis, Lowdin population analysis, and natural bond orbital analysis. Classical GCMC and MD simulations of RDX in IRMOF-10 are performed using 15 combinations of charge sources of RDX and IRMOF-10. As the charge distributions vary, interaction potential energies, the adsorption loading, and the self-diffusivities are significantly different. None of the 15 combinations are able to quantitatively capture the dependence of the energy of adsorption on local configuration of RDX as observed in the QM calculations. We observe changes in the charge distributions of RDX and IRMOF-10 with the introduction of an RDX molecule into the cage. We also observe a large dispersion contribution to the interaction energy from QM calculations that is not reproduced in the classical simulations, indicating that the source of discrepancy may not lie exclusively with the assignment of charges.

Calculated electronic and magnetic structure of screw dislocations in alpha iron

Local atomic magnetic moments in crystalline Fe are perturbed by the presence of dislocations. The effects are most pronounced near the dislocation core and decay slowly as the strain field of the dislocation decreases with distance. We have calculated local moments using the locally self-consistent multiple scattering (LSMS) method for a supercell containing a screw-dislocation quadrupole. Finite size effects are found to be significant indicating that dislocation cores affect the electronic structure and magnetic moments of neighboring dislocations. The influence of neighboring dislocations points to a need to study individual dislocations from first principles just as they appear amid surrounding atoms in large-scale classical force field simulations. An approach for the use of the LSMS to calculate local moments in subvolumes of large atomic configurations generated in the course of classical molecular dynamics simulation of dislocationdynamics is discussed.

Coarse Grained Approach to First Principles Modeling of Radiation Cascade in Large Fe Supercells

Classical Molecular Dynamics (MD) simulations characterizing dislocations and radiation damage typically treat 105-107 atoms. First principles techniques employed to understand systems at an atomistic level are not practical for such large systems consisting of millions of atoms. We present an efficient coarse grained (CG) approach to calculate local electronic and magnetic properties of large MD-generated structures from the first principles. Local atomic magnetic moments in crystalline Fe are perturbed by the presence of radiation generated vacancies and interstitials. The effects are most pronounced near the defect cores and decay slowly as the strain field of the defects decrease with distance. We develop the CG technique based on the Locally Self-consistent Multiple Scattering (LSMS) method that exploits the near-sightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. The calculations result in local moments near the defect centers with first-principles accuracy, while capturing coarse-grained details of local moments at greater length scales. This CG approach makes these large scale structures amenable to first principles study.

Lithium diffusion at Si-C interfaces in silicon-graphene composites

Models of intercalated Li and its diffusion in Si-Graphene interfaces are investigated using density functional theory. Results suggest that the presence of interfaces alters the energetics of Li binding and diffusion significantly compared to bare Si or Graphene surfaces. Our results show that cavities along reconstructed Si surface provide diffusion paths for Li. Diffusion barriers calculated along these cavities are significantly lower than penetration barriers to bulk Si. Interaction with Si surface results in graphene defects, creating Li diffusion paths that are confined along the cavities but have still lower barrier than in bulk Si.

Exact Enumeration of the Phase Space of an Ising Model of Ni

Exact evaluations of partition functions are generally prohibitively expensive due to exponential growth of phase space with the number of degrees of freedom. For an sing model with sites the number of possible states is requiring the use of better scaling methods such as importance sampling Monte-Carlo calculations for all but the smallest systems. Yet the ability to obtain exact solutions for as large as possible systems can provide important benchmark results and opportunities for unobscured insight into the underlying physics of the system. Here we present an sing model for the magnetic sublattices of the important magneto-caloric material Ni2MnGa and use an exact enumeration algorithm to calculate the number of states for each energy and sublattice magnetizations MNi and MMn. This allows the efficient calculation of the partition function and derived thermodynamic quantities such as specific heat and susceptibility. Utilizing the jaguarpf system at Oak Ridge we are able to calculate for systems of up to 48 sites, which provides important insight into the mechanism for the large magnet-caloric effect in Ni2MnGa as well as an important benchmark for Monte-Carlo (esp. Wang-Landau method).