Blas P. Uberuaga

A climbing image nudged elastic band method for finding saddle points and minimum energy paths

A modification of the nudged elastic band method for finding minimum energy paths is presented. One of the images is made to climb up along the elastic band to converge rigorously on the highest saddle point. Also, variable spring constants are used to increase the density of images near the top of the energy barrier to get an improved estimate of the reaction coordinate near the saddle point. Applications to CH4 dissociative adsorption on Ir~111! and H2 on Si~100! using plane wave based density functional theory are presented.

Efficient Annealing of Radiation Damage Near Grain Boundaries via Interstitial Emission

Preventing Radiation Damage Inside a nuclear reactor, long-term exposure to radiation causes structural damage and limits the lifetimes of the reactor components. Bai et al. (p. 1631; see the Perspective by Ackland) now show, using three simulation methods able to cover a wide range of time and length scales, that grain boundaries in copper can act as sinks for radiation-induced defects. The boundaries are able to store up defects, in the form of interstitials, which subsequently annihilate with vacancies in the bulk. This recombination mechanism has a lower energy barrier than the bulk equivalent, and so provides a lower-cost route for the copper to self-heal. Simulations show that grain boundaries store and annihilate radiation-induced defects in copper. Although grain boundaries can serve as effective sinks for radiation-induced defects such as interstitials and vacancies, the atomistic mechanisms leading to this enhanced tolerance are still not well understood. With the use of three atomistic simulation methods, we investigated defect–grain boundary interaction mechanisms in copper from picosecond to microsecond time scales. We found that grain boundaries have a surprising “loading-unloading” effect. Upon irradiation, interstitials are loaded into the boundary, which then acts as a source, emitting interstitials to annihilate vacancies in the bulk. This unexpected recombination mechanism has a much lower energy barrier than conventional vacancy diffusion and is efficient for annihilating immobile vacancies in the nearby bulk, resulting in self-healing of the radiation-induced damage.

Vacancy-mediated dopant diffusion activation enthalpies for germanium

Electronic structure calculations are used to predict the activation enthalpies of diffusion for a range of impurity atoms (aluminium, gallium, indium, silicon, tin, phosphorus, arsenic, and antimony) in germanium. Consistent with experimental studies, all the impurity atoms considered diffuse via their interaction with vacancies. Overall, the calculated diffusion activation enthalpies are in good agreement with the experimental results, with the exception of indium, where the most recent experimental study suggests a significantly higher activation enthalpy. Here, we predict that indium diffuses with an activation enthalpy of 2.79eV, essentially the same as the value determined by early radiotracer studies.

Synchronization of trajectories in canonical molecular-dynamics simulations: Observation, explanation, and exploitation

For two methods commonly used to achieve canonical-ensemble sampling in a molecular-dynamics simulation, the Langevin thermostat and the Andersen [H. C. Andersen, J. Chem. Phys. 72, 2384 (1980)] thermostat, we observe, as have others, synchronization of initially independent trajectories in the same potential basin when the same random number sequence is employed. For the first time, we derive the time dependence of this synchronization for a harmonic well and show that the rate of synchronization is proportional to the thermostat coupling strength at weak coupling and inversely proportional at strong coupling with a peak in between. Explanations for the synchronization and the coupling dependence are given for both thermostats. Observation of the effect for a realistic 97-atom system indicates that this phenomenon is quite general. We discuss some of the implications of this effect and propose that it can be exploited to develop new simulation techniques. We give three examples: efficient thermalization (a concept which was also noted by Fahy and Hamann [S. Fahy and D. R. Hamann, Phys. Rev. Lett. 69, 761 (1992)]), time-parallelization of a trajectory in an infrequent-event system, and detecting transitions in an infrequent-event system.

Vacancy-arsenic clusters in germanium

Electronic structure calculations are used to investigate the structures and relative energies of defect clusters formed between arsenic atoms and lattice vacancies in germanium and, for comparison, in silicon. It is energetically favorable to form clusters containing up to four arsenic atoms tetrahedrally coordinated around a vacancy. Using mass action analysis, the relative concentrations of arsenic atoms in different vacancy-arsenic clusters, unbound arsenic atoms, and unbound vacancies are predicted. At low temperatures the four arsenic-vacancy cluster is dominant over unbound vacancies while at higher temperatures unbound vacancies prevail. In terms of concentration, no intermediate size of cluster is ever of significance.

Chapter 4 Accelerated Molecular Dynamics Methods: Introduction and Recent Developments

Abstract Because of its unrivaled predictive power, the molecular dynamics (MD) method is widely used in theoretical chemistry, physics, biology, materials science, and engineering. However, due to computational cost, MD simulations can only be used to directly simulate dynamical processes over limited timescales (e.g., nanoseconds or at most a few microseconds), even though the simulation of nonequilibrium processes can often require significantly longer timescales, especially when they involve thermal activation. In this paper, we present an introduction to accelerated molecular dynamics, a class of methods aimed at extending the timescale range of molecular dynamics, sometimes up to seconds or more. The theoretical foundations underpinning the different methods (parallel replica dynamics, hyperdynamics, and temperature-accelerated dynamics) are first discussed. We then discuss some applications and recent advances, including super-state parallel replica dynamics, self-learning hyperdynamics, and spatially parallel temperature-accelerated dynamics.

Carbon, dopant, and vacancy interactions in germanium

Electronic structure calculations have been used to study the interaction of carbon with isolated substitutional dopants (boron, phosphorus, or arsenic), vacancies, and dopant-vacancy pairs in germanium. For comparison, equivalent defects were examined in silicon. The calculations employed a plane-wave basis set and pseudopotentials within the generalized gradient approximation of density functional theory. The results predict a range of different association preferences, with carbon being strongly bound in some cases and unbound in others. For example, in germanium, the carbon-vacancy cluster is weakly bound whereas in silicon it is more strongly bound. Conversely, dopant-carbon pairs are not stable in either germanium or silicon compared to their isolated components. If, however, they are formed during implantation, they will act as strong vacancy traps. Details of clusters comprised of a dopant, carbon, and vacancy are also discussed with respect to their formation by the association of a vacancy or cl...

The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus, sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary.

Machine learning bandgaps of double perovskites.

The ability to make rapid and accurate predictions on bandgaps of double perovskites is of much practical interest for a range of applications. While quantum mechanical computations for high-fidelity bandgaps are enormously computation-time intensive and thus impractical in high throughput studies, informatics-based statistical learning approaches can be a promising alternative. Here we demonstrate a systematic feature-engineering approach and a robust learning framework for efficient and accurate predictions of electronic bandgaps of double perovskites. After evaluating a set of more than 1.2 million features, we identify lowest occupied Kohn-Sham levels and elemental electronegativities of the constituent atomic species as the most crucial and relevant predictors. The developed models are validated and tested using the best practices of data science and further analyzed to rationalize their prediction performance.

Radiation effects in solids

1. Kinetic Monte Carlo A.F. Voter.- 2. Accelerated Molecular Dynamics Methods B.P. Uberuaga, A.F. Voter.- 3. Radiation Induced Structural Changes through In-Situ TEM observations C. Kinoshita.- 4. Radiation Damage from Different Particle Types G.S. Was, T.R. Allen.- 5. High Dose Radiation Effects in Steels T.R. Allen.- 6. Radiation-Enhanced Diffusion and Radiation-Induced Segregation T.R. Allen, G.S. Was.- 7. The Kinetics of Radiation-Induced Point Defect Aggregation and Metallic Colloid Formation in Ionic Solids E.A. Kotomin, A.I. Popov.- 8. Microstructural Evolution off Irradiated Ceramics C. Kinoshita.- 9. Optical & Scintillation Properties of Nonmetals: Inorganic Scintillators for Radiation Detectors V.N. Makhov.- 10. Radiation-Induced Phase Transitions P.M. Ossi.- 11. Introduction to Mathematical Models for Irradiation Induced Phase Transformations K.E. Sickafus.- 12. Amorphous Systems and Amorphization H. Bernas.- 13. Ion Beam Mixing M. Nastasi, J.W. Mayer.- 14. Radiation Effects in Nuclear Fuels H. Matzke.- 15. Role of Irradiation in Stress Corrosion Cracking G.S. Was.- 16. Ion Beam Synthesis and Tailoring of Nanostructures H. Bernas, R. Espiau de Lamaestre.- 17. Residual Stress Evolution During Energetic Particle Bombardment of Thin Films A. Misra, M. Nastasi.- 18. Perovskite-Based Colossal Magneto-Resistance Materials and their Irradiation Studies: A Review R. Kumar et al.- 19. Exposure of Bone to Ionizing Radiation L. Kubisz.- Index.-