The bond-order potential (BOP) for transition metals is a real-space semiempirical description of interactions between the atoms, which is based on the tight-binding approximation and the d-band model. This scheme provides a direct bridge between the electronic level modeling and the atomistic modeling, where the electronic degrees of freedom have been coarse grained into many-body interatomic potentials. In this paper we construct BOP in which both the attractive and the repulsive contributions to the binding energy are environmentally dependent due to both the nonorthogonality of the orbitals and the breathing of the screening charges. The construction of the BOP is described and tested in detail. First, the energies of alternative crystal structures (A15, fcc, hcp, simple cubic) are calculated and compared with those evaluated ab initio. The transferability of the BOP to atomic configurations that deviate significantly from the bcc lattice is studied by computing the energies along tetragonal, trigonal, and hexagonal transformation paths. Next, the phonon spectra are evaluated for several symmetrical crystallographic directions and compared with available experiments. All these calculations highlight the importance of directional bonding and the investigation of phonons demonstrates that the environmental dependence of the bond integrals is crucial for the phonons of the N branch not to be unphysically soft. Finally, the constructed BOP was applied in the modeling of the core structure and glide of the 1/2⟨ ⟨111⟩ ⟩ screw dislocation. The calculated structure of the core agrees excellently with that found in the recent ab initio calculations and the observed glide behavior not only agrees with available ab initio data but is in agreement with many experimental observations and explains the primary reason for the breakdown of the Schmid law in bcc metals. The bond-order potential ͑BOP͒ for transition metals is a real-space semiempirical description of interactions between the atoms, which is based on the tight-binding approximation and the d-band model. This scheme provides a direct bridge between the electronic level modeling and the atomistic modeling, where the electronic degrees of freedom have been coarse grained into many-body interatomic potentials. In this paper we construct BOP in which both the attractive and the repulsive contributions to the binding energy are environmentally dependent due to both the nonorthogonality of the orbitals and the breathing of the screening charges. The construction of the BOP is described and tested in detail. First, the energies of alternative crystal structures ͑A15, fcc, hcp, simple cubic͒ are calculated and compared with those evaluated ab initio. The transferability of the BOP to atomic configurations that deviate significantly from the bcc lattice is studied by computing the energies along tetragonal, trigonal, and hexagonal transformation paths. Next, the phonon spectra are evaluated for several symmetrical crystallographic directions and compared with av...

We present a bond-order potential (BOP) for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling methods the BOPs thus provide a direct bridge between electronic-structure and atomistic techniques. Two variants of the BOP were constructed and extensively tested against accurate first-principles methods in order to assess the potentials' reliability and applicability. A comparison of the BOP with a central-force potential is used to demonstrate that a correct description of directional mixed covalent and metallic bonds is crucial for a successful and fully transferable model. The potentials are applied in studies of low-index surfaces, symmetrical tilt grain boundaries, and dislocations. We present a bond-order potential ͑BOP͒ for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling methods the BOPs thus provide a direct bridge between electronic-structure and atomistic techniques. Two variants of the BOP were constructed and extensively tested against accurate first-principles methods in order to assess the potentials' reliability and applicability. A comparison of the BOP with a central-force potential is used to demonstrate that a correct description of directional mixed covalent and metallic bonds is crucial for a successful and fully transferable model. The potentials are applied in studies of low-index surfaces, symmetrical tilt grain boundaries, and dislocations.

The atomic cluster expansion is a general polynomial expansion of the atomic energy in multi-atom basis functions. Here we implement the atomic cluster expansion in the performant C++ code that is suitable for use in large-scale atomistic simulations. We briefly review the atomic cluster expansion and give detailed expressions for energies and forces as well as efficient algorithms for their evaluation. We demonstrate that the atomic cluster expansion as implemented in shifts a previously established Pareto front for machine learning interatomic potentials toward faster and more accurate calculations. Moreover, general purpose parameterizations are presented for copper and silicon and evaluated in detail. We show that the Cu and Si potentials significantly improve on the best available potentials for highly accurate large-scale atomistic simulations.

We present a magnetic bond-order potential (BOP) that is able to provide a correct description of both directional covalent bonds and magnetic interactions in iron. This potential, based on the tight binding approximation and the Stoner model of itinerant magnetism, forms a direct bridge between the electronic-structure and the atomistic modeling hierarchies. Even though BOP calculations are computationally more demanding than those using common empirical potentials, the formalism can be used for studies of complex defect configurations in large atomic ensembles exceeding 10(5) atoms. Our studies of dislocations in α-Fe demonstrate that correct descriptions of directional covalent bonds and magnetism are crucial for a reliable modeling of these defects.

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A correct description of hydrogen diffusion and trapping is the prerequisite for an understanding of the phenomenon of hydrogen embrittlement. In this study, we carried out extensive first-principles calculations based on density functional theory to investigate the interaction of H with TiC precipitates that are assumed to be efficient trapping agents mitigating HE in advanced high-strength steels. We found that there exists a large variety of possible trapping sites for H associated with different types of interfaces between the TiC particle and the Fe matrix, with misfit dislocations and other defects at these interfaces, and with carbon vacancies in TiC. The most efficient trapping by more than 1 eV occurs at carbon vacancies in the interior of TiC particles. However, these traps are difficult to populate at ambient temperatures since the energy barrier for H entering the particles is high. H trapping at the semicoherent interfaces between the TiC particles and the Fe matrix is moderate, ranging from 0.3 to 0.5 eV. However, a sufficiently large concentration of the carbide particles can significantly reduce the amount of H segregated at dislocation cores in the Fe matrix. A systematic comparison of the obtained theoretical results with available experimental observations reveals a consistent picture of hydrogen trapping at the TiC particles that is expected to be qualitatively valid also for other carbide precipitates with the rock-salt crystal structure.

Halogenated argyrodites Li6PS5Br, Li6PS5Cl, and Li6PS5I exhibit large differences in the measured Li ionic conductivities. Crystallographic analysis has shown that these differences may be related to occupations of specific Wyckoff sites in different argyrodite types, but detailed understanding of the relationship between the atomic structure and operating diffusion mechanisms is still lacking. In this work, we employed ab initio molecular dynamics simulations to calculate the Li diffusivity for different argyrodite structure types. Our calculations show that the Li diffusivity does not depend implicitly on the type of halogen but is rather governed by the degree of structural disorder. Assuming disordered structures to arise naturally from the ordered structure type by thermally activated antisite defects, we are able to explain the degree of disorder found for the different types of halogens from the calculated defect formation energies. Comparing the calculated formation energies to the ionic radii of the halogen atoms, we find a strong correlation between the radii and energies required for introducing the antisite defects.