<i>Ab Initio</i>Molecular Dynamics with Excited Electrons

Alavi, Ali; Kohanoff, Jorge; Parrinello, Michele; Frenkel, Daan

doi:10.1103/physrevlett.73.2599

Cited by 239 publications

(180 citation statements)

References 21 publications

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“…A detailed analysis of the electronic structure for selected geometries ͑snapshots͒ has been carried out with the Lanczos diagonalization scheme and the free energy functional of Alavi et al ͑T = 1000 K͒. 26 Memory of the crystalline starting structure was erased by starting the simulations at 3000 K ͑liquid͒, followed by gradual cooling over 42 ps to the melting point ͑900 K͒. The first data collection was performed for 21 ps at 900 K, followed by cooling to 300 K over 139 ps.…”

Section: A Density Functional Calculationsmentioning

confidence: 99%

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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Phase-change materials are of immense importance for optical recording and computer memory, but the structure of the amorphous phases and the nature of the phase transition in the nanoscale bits pose continuing challenges. Massively parallel density functional simulations have been used to characterize the amorphous structure of the prototype materials Ge 2 Sb 2 Te 5 and GeTe. In both, there is long-ranged order among Te atoms and the crucial structural motif is a four-membered ring with alternating atoms of types A ͑Ge and Sb͒ and B ͑Te͒, an "ABAB square." The rapid amorphous-to-crystalline phase change is a reorientation of disordered ABAB squares to form an ordered lattice. There are deviations from the "8 − N rule" for coordination numbers, with Te having near threefold coordination. Ge atoms are predominantly fourfold coordinated, but-contrary to recent speculation-tetrahedral coordination is found in only approximately one-third of the Ge atoms. The average coordination number of Sb atoms is 3.7, and the local environment of Ge and Sb is usually "distorted octahedral" with AB separations from 3.2 to 4 Å in the first coordination shell. The number of A -A bonds is significantly greater in GeTe than in Ge 2 Sb 2 Te 5 . Vacancies ͑voids͒ in the disordered phases of these materials provide the necessary space for the phase transitions to take place. The vacancy concentration in Ge 2 Sb 2 Te 5 ͑11.8%͒ is greater than in GeTe ͑6.4%͒, which is consistent with the better phase-change performance of the former.

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Section: A Density Functional Calculationsmentioning

confidence: 99%

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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“…The most elegant solution would be to treat electrons in the grand canonical ensemble, that is, to allow the number of electrons to change during the electronic structure self-consistency loop, which is equivalent to connecting the system to a fictitious potentiostat. 5,6 Unfortunately, convergence of the grand canonical Kohn-Sham equations is significantly slower than with a fixed number of electrons, limiting the method to small system sizes. Other suggested methods rely on extrapolating calculated energies from canonical, constant charge DFT simulations to constant electrode potential.…”

Section: Introductionmentioning

confidence: 99%

“…Several methods [5][6][7][8][9][10][11][12][13] have been proposed for correcting quantum chemical calculations to constant electrode potential. The most elegant solution would be to treat electrons in the grand canonical ensemble, that is, to allow the number of electrons to change during the electronic structure self-consistency loop, which is equivalent to connecting the system to a fictitious potentiostat.…”

Section: Introductionmentioning

confidence: 99%

Ab Initio Electrochemistry: Exploring the Hydrogen Evolution Reaction on Carbon Nanotubes

Holmberg

Laasonen

2015

J. Phys. Chem. C

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Density functional theory (DFT) was employed to investigate the hydrogen evolution reaction (HER) on pristine and nitrogen doped carbon nanotubes (CNTs) in acidic solution. As the reaction is an electrocatalytic surface reaction, an accurate description of HER requires performing simulations under constant electrode potential conditions. To this end, we examined HER at several electrode charges allowing us to determine grand canonical activation energies as a continuous function of electrode potential. By studying the elementary steps of HER -the Volmer, Tafel and Heyrovsky reactions -and by considering hydrogen coverage effects, we found that the Volmer-Heyrovsky mechanism is the predominant HER mechanism on CNTs with the Heyrovsky step being rate-determining. Our results indicated that CNTs are electrochemically active towards HER, which seen as a decrease in activation energies with growing electrode potential, but that the activity is lower than on platinum matching experimental data.Substitutional nitrogen doping did not improve HER activity, and further research is required to determine which nitrogen configurations contribute to the enhanced catalytic activity observed for experimental NCNTs.

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“…Contemporary computations on WDM [5][6][7][8][9][10][11][12][13][14][15][16][17][18] are dominated by use of the Kohn-Sham (KS) realization of thermal density functional theory (DFT) [19][20][21][22][23][24][25] to generate a potential surface for ionic motion (treated classically). The majority of such calculations use approximate ground-state exchange-correlation (XC) functionals, E xc , with the temperature dependence of the XC free energy picked up implicitly from the T-dependence of the density n(r, T).…”

Section: Introductionmentioning

confidence: 99%

Temperature-dependent behavior of confined many-electron systems in the Hartree-Fock approximation

2012

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Many-electron systems confined at substantial finite temperatures and densities present a major challenge to density functional theory. In particular, there is comparatively little systematic knowledge about the behavior of free-energy density functionals for temperatures and pressures of interest, for example, in the study of warm dense matter. As with ground-state functionals, development of approximate free-energy functionals is faced with significant needs for reliable assessment and calibration data. Here we address, in part, this need for detailed results on well-characterized systems. We present results on a comparatively simple, well-defined, computationally feasible but previously unexplored model, the thermal Hartree-Fock approximation. We discuss the main technical tasks (defining a suitable basis and evaluation of the required matrix elements) and give an illustrative initial application which probes both the content of the model and the solution techniques: a system of eight one-electron atoms with nuclei at fixed, arbitrary positions in a hard-walled box. Even this simple system produces physical behavior different from that produced by simple ground state density functionals used at finite temperature (a common approximation in the study of WDM).

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Ab InitioMolecular Dynamics with Excited Electrons

Cited by 239 publications

References 21 publications

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

Ab Initio Electrochemistry: Exploring the Hydrogen Evolution Reaction on Carbon Nanotubes

Temperature-dependent behavior of confined many-electron systems in the Hartree-Fock approximation

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