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Full-Text Articles in Physics

Adsorption And Diffusion Of Ru Adatoms On Ru(0001)-Supported Graphene: Large-Scale First-Principles Calculations, Yong Han, James W. Evans Oct 2015

Adsorption And Diffusion Of Ru Adatoms On Ru(0001)-Supported Graphene: Large-Scale First-Principles Calculations, Yong Han, James W. Evans

Physics and Astronomy Publications

Large-scale first-principles density functional theory calculations are performed to investigate the adsorption and diffusion of Ru adatoms on monolayer graphene (G) supported on Ru(0001). The G sheet exhibits a periodic moiré-cell superstructure due to lattice mismatch. Within a moirécell, there are three distinct regions: fcc, hcp, and mound, in which the C6-ring center is above a fcc site, a hcp site, and a surface Ru atom of Ru(0001), respectively. The adsorption energy of a Ru adatom is evaluated at specific sites in these distinct regions. We find the strongest binding at an adsorption site above a C atom ...


Nudged-Elastic Band Method With Two Climbing Images: Finding Transition States In Complex Energy Landscapes, Nikolai A. Zarkevich, Duane D. Johnson Jan 2015

Nudged-Elastic Band Method With Two Climbing Images: Finding Transition States In Complex Energy Landscapes, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along ...


Magneto-Structural Transformations Via A Solid-State Nudged Elastic Band Method: Application To Iron Under Pressure, Nikolai A. Zarkevich, Duane D. Johnson Jan 2015

Magneto-Structural Transformations Via A Solid-State Nudged Elastic Band Method: Application To Iron Under Pressure, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter, in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volumecollapse during the pressure-induced transformation in iron—from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface ...