François Bottin

Large scale ab initio calculations based on three levels of parallelization

Abstract We suggest and implement a parallelization scheme based on an efficient multiband eigenvalue solver, called the locally optimal block preconditioned conjugate gradient ( lobpcg ) method, and using an optimized three-dimensional (3D) fast Fourier transform (FFT) in the ab initio plane-wave code abinit . In addition to the standard data partitioning over processors corresponding to different k-points, we introduce data partitioning with respect to blocks of bands as well as spatial partitioning in the Fourier space of coefficients over the plane waves basis set used in abinit . This k-points-multiband-FFT parallelization avoids any collective communications on the whole set of processors relying instead on one-dimensional communications only. For a single k-point, super-linear scaling is achieved for up to 100 processors due to an extensive use of hardware-optimized blas , lapack and scalapack routines, mainly in the lobpcg routine. We observe good performance up to 200 processors. With 10 k-points our three-way data partitioning results in linear scaling up to 1000 processors for a practical system used for testing.

Strong isotope effect in phase II of dense solid hydrogen and deuterium.

Quantum nuclear zero-point motions in solid H(2) and D(2) under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well defined in phase II of D(2), while solid H(2) exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the (bc) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation.

Oxide ion and proton transport in Gd-doped barium cerate: a combined first-principles and kinetic Monte Carlo study

Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 are investigated by first-principles density-functional calculations and kinetic Monte Carlo simulations. Behind the apparent complexity of the energy landscape (related to the low-symmetry of the system), general tendencies governing the energy barriers can be extracted. In particular, the set of barriers is tested with respect to the Bell–Evans–Polanyi (BEP) principle that relates the activation energy Ea of a series of similar chemical reactions to their reaction enthalpies ΔH. This rule is found poorly satisfied in the case of oxide ion migration, but much better satisfied for proton hopping mechanisms. Protonic reorientations, by contrast, do not obey the BEP rule. Kinetic Monte Carlo simulations give insight into the macroscopic transport properties. We observe that dopants act as traps for oxygen vacancies and slow down their motion, whereas their effect on the protonic diffusion coefficient is more complex. This is related to the fact that a two-state picture roughly applies to the oxygen vacancy energy landscape, while it does not apply to the protonic one. Oxide ion migration exhibits strong anisotropy, the motion of the oxygen vacancies being favored in the equatorial planes, while protonic diffusion is found more isotropic.

Iron under conditions close to the α − γ − ϵ triple point

The stability domains and equations of state of α-Fe, ϵ-Fe, and γ-Fe have been measured using X-ray diffraction under conditions close to their triple point: 7 ≤ P ≤ 20 GPa and 480 ≤ T ≤ 820 K. Special attention was paid to ensure the hydrostatic compression of the sample, which was a single crystal at start. Narrow α − γ and α − ϵ coexistence domains were observed, while the γ − ϵ transformation appeared sluggish. The triple point is measured at 8.7 ± 1.0 GPa and 750 ± 30 K. Anharmonic effects are evidenced in the equation of state of ϵ-Fe and partly reproduced using ab initio molecular dynamics simulations.The stability domains and equations of state of α-Fe, ϵ-Fe, and γ-Fe have been measured using X-ray diffraction under conditions close to their triple point: 7 ≤ P ≤ 20 GPa and 480 ≤ T ≤ 820 K. Special attention was paid to ensure the hydrostatic compression of the sample, which was a single crystal at start. Narrow α − γ and α − ϵ coexistence domains were observed, while the γ − ϵ transformation appeared sluggish. The triple point is measured at 8.7 ± 1.0 GPa and 750 ± 30 K. Anharmonic effects are evidenced in the equation of state of ϵ-Fe and partly reproduced using ab initio molecular dynamics simulations.