arXiv subjects

Axel van de Walle

Publications and source records attributed to Axel van de Walle.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Contributions to Phase Stability in the Co-Ni Binary: A First-Principles CALPHAD Study

We propose a simple method to employ ab-initio calculations to determine magnetic contributions to free energy of alloys. Validation on the Co-Ni binary demonstrates that this ab initio approach reproduces experimental FCC-HCP phase equilibria while providing physically transparent, structure-dependent magnetic parameters suitable for multicomponent extrapolation. Our results demonstrate that physically grounded magnetic parametrization, derived directly from electronic structure calculations, enables predictive phase diagram modeling for magnetic alloy systems.

cond-mat.mtrl-sci↗

arXiv2026

SimplySQS: An Automated and Reproducible Workflow for Special Quasirandom Structure Generation with ATAT

The special quasirandom structure (SQS) method is widely used for modeling disordered materials under periodic boundary conditions, with the ATAT mcsqs module being one of the most established implementations. However, SQS generation with mcsqs typically relies on manual preparation of input files, ad hoc execution scripts, and post-processing steps, which introduces user-dependent errors and limits reproducibility. Here, we present SimplySQS (https://simplysqs.com), an automated and reproducible workflow for SQS generation that is delivered through an online, interactive interface. SimplySQS guides users through structure import, compositional and supercell definition, and cluster parameter selection, while automatically generating all required ATAT input files and a single all-in-one execution script that encapsulates the complete search process. By standardizing input preparation, execution, and output analysis, the framework minimizes errors associated with manual file handling and enables consistent reproducibility of SQS searches. The workflow is demonstrated on the Pb1-xSrxTiO3 (PSTO, including PbTiO3 (PTO) and SrTiO3 (STO)) perovskite system. SQSs spanning the entire concentration range were generated using a single automated bash script produced by SimplySQS, after which all resulting structures were subjected to geometry optimization using a universal machine-learning interatomic potential (MACE MATPES-r2SCAN-0). This approach reliably reproduced the experimentally observed cubic-to-tetragonal transition near x = 0.5, with lattice parameters deviating by less than 1 % in the cubic region (x > 0.5) and less than 4 % in the tetragonal region (x < 0.5). Overall, SimplySQS transforms SQS generation with ATAT into an intuitive, reproducible, and systematic framework for modeling disordered materials.

cond-mat.mtrl-sci↗

arXiv2026

Pilot-waves and copilot-particles: A nonstochastic approach to objective wavefunction collapse

We seek an extension to Schrodinger's equation that incorporates the macroscopic measurement-induced wavefunction collapse phenomenon. We find that a suitable hybrid between two leading approaches, the Bohm-de Broglie pilot-wave and objective collapse theories, accomplishes this goal in a way that is consistent with Born's rule. Our theory posits that the Bohmian particle is guided by the wavefunction and, conversely, the wavefunction gradually localizes towards the particle's position. As long as the particle can visit any state, as in a typical microscopic system, the localization effect does not favor any particular quantum state and, on average, the usual Schrodinger-like time evolution results. However, when the wavefunction develops spatially well-separated lobes, as would happen during a macroscopic measurement, the Bohmian particle can remain trapped in one lobe, which causes the wavefunction to eventually localizes. This proposed loss of ergodicity mechanism recasts one of the foundational postulate of quantum mechanics as a emergent feature and has important implications regarding the feasibility of large-scale quantum computing.

cond-mat.stat-mech↗

arXiv2026

A Nonlocal Orientation Field Phase-Field Model for Misorientation- and Inclination- Dependent Grain Boundaries

We propose to incorporate grain boundary (GB) anisotropy in phase-field modeling by extending the standard partial differential equations formulation to include a nonlocal functional of an orientation field. Regardless of the number of grains in the simulation, the model uses a single orientation field and incorporates grain misorientation and inclination information obtained from sampling the orientation field at optimized locations in the vicinity of the grain boundary. The formalism enables simple and precise tuning of GB energy anisotropy while reducing an extensive fitting procedure. The functional includes an explicit GB anisotropy function to control the GB energy as a function of both misorientation and inclination. The model is validated by reproducing the linear grain growth rate, Wulff shapes with varying misorientations and anisotropic coefficients, and analytical equilibrium dihedral angles at triple junctions. Polycrystalline simulations demonstrate grain growth, coalescence, triple junction behavior, and the influence of anisotropy on grain morphology.

cond-mat.mtrl-sci↗

arXiv2025

Multicomponent multisublattice alloys, nonconfigurational entropy and other additions to the Alloy Theoretic Automated Toolkit

A number of new functionalities have been added to the Alloy Theoretic Automated Toolkit (ATAT) since it was last reviewed in this journal in 2002. ATAT can now handle multicomponent multisublattice alloy systems, nonconfigurational sources of entropy (e.g. vibrational and electronic entropy), Special Quasirandom Structures (SQS) generation, tensorial cluster expansion construction and includes interfaces for multiple atomistic or ab initio codes. This paper presents an overview of these features geared towards the practical use of the code. The extensions to the cluster expansion formalism needed to cover multicomponent multisublattice alloys are also formally demonstrated.

cond-mat.mtrl-sci↗

arXiv2025

Density Functional Theory ToolKit (DFTTK) to Automate First-Principles Thermodynamics via the Quasiharmonic Approximation

The Helmholtz energy is a key thermodynamic quantity representing available energy to do work at a constant temperature and volume. Despite a well-established methodology from first-principles calculations, a comprehensive tool and database are still lacking. To address this gap, we developed an open-source Density Functional Theory Tool Kit (DFTTK), which automates first-principles thermodynamics using the quasiharmonic approximation (QHA) for Helmholtz energy predictions. This Python-based package provides a solution for automating the calculation and analysis of various contributions to Helmholtz energy, including the static total energy contributions at 0 K in terms of DFT-based energy-volume curves, vibrational contributions from the Debye-Gruneisen model and phonons, and thermal electronic contributions via the electronic density of states. The QHA is also implemented to calculate the Gibbs energy and associated properties at constant temperature and pressure. The present work demonstrates DFTTK's capabilities through case studies on a simple FCC Al and various collinear magnetic configurations of Invar Fe3Pt, where DFTTK enumerates all unique configurations and their associated multiplicities. DFTTK is freely available on GitHub, and its modular design allows for the easy addition of new workflows.

cond-mat.mtrl-sci↗

arXiv2025

Computational Study of Density Fluctuation-Induced Shear Bands Formation in Bulk Metallic Glasses

Seemingly identical Bulk Metallic Glasses (BMG) often exhibit strikingly different mechanical properties despite having the same composition and fictive temperature. A postulated mechanism underlying these differences is the presence of "defects". Here we investigate this hypothesis through the study of the effect of density fluctuations on shear band formation under an applied stress. We find that the critical shear stress is strongly dependent on the magnitude and size of the fluctuations. This finding also elucidates why, historically, critical shear stresses obtained in simulations have differed so much from those found experimentally, as typical simulations setups might favor unrealistically uniform geometries.

cond-mat.dis-nn↗

arXiv2025

Soliquidy: a descriptor for atomic geometrical confusion

Tailoring material properties often requires understanding the solidification process. Herein, we introduce the geometric descriptor Soliquidy, which numerically captures the Euclidean transport cost between the translationally disordered versus ordered states of a materials. As a testbed, we apply Soliquidy to the classification of glass-forming metal alloys. By extending and combining an experimental library of metallic thin-films (glass/no-glass) with the aflow.org computational database (geometrical and energetic information of mixtures) we found that the combination of Soliquity and formation enthalpies generates an effective classifier for glass formation. Such classifier is then used to tackle a public dataset of metallic glasses showing that the glass-agnostic assumptions of Soliquity can be useful for understanding kinetically-controlled phase transitions.

cond-mat.mtrl-sci↗

arXiv2023

Special Glass Structures for First Principles Studies of Bulk Metallic Glasses

The atomic-level structure of bulk metallic glasses is a key determinant of their properties. An accurate representation of amorphous systems in computational studies has traditionally required large supercells that are unfortunately computationally demanding to handle using the most accurate ab initio calculations. To address this, we propose to specifically design small-cell structures that best reproduce the local geometric descriptors (e.g., pairwise distances or bond angle distributions) of a large-cell simulation. We rely on molecular dynamics (MD) driven by empirical potentials to generate the target descriptors, while we use reverse Monte Carlo (RMC) methods to optimize the small-cell structure. The latter can then be used to determine mechanical and electronic properties using more accurate electronic structure calculations. The method is implemented in the Metallic Amorphous Structures Toolkit (MAST) software package.

cond-mat.mtrl-sci↗

arXiv2020

Re-entrant melting of sodium, magnesium, and aluminum: General trend

Re-entrant melting (in which a substance's melting point starts to decrease beyond a certain pressure) is believed to be an unusual phenomenon. Among the elements, it has so far only been observed in a very limited number of species, e.g., the alkali metals. Our density functional theory calculations reveal that this behavior actually extends beyond alkali metals to include magnesium, which also undergoes re-entrant melting, though at the much higher pressure of ~300 GPa. We find that the origin of re-entrant melting is the faster softening of interatomic interactions in the liquid phase than in the solid, as pressure rises. We propose a simple approach to estimate pressure-volume relations and show that this characteristic softening pattern is widely observed in metallic elements. We verify this prediction in the case of aluminum by finding re-entrant melting at ~4000 GPa. These results suggest that re-entrant melting may be a more universal feature than previously thought.

cond-mat.mtrl-sci↗

arXiv2019

A simple local expression for the prefactor in transition state theory

We present a simple and accurate computational technique to determine the frequency prefactor in harmonic transition state theory without necessitating full phonon density of states (DOS) calculations. The atoms in the system are partitioned into an "active region", where the kinetic process takes place, and an "environment" surrounding the active region. It is shown that the prefactor can be obtained by a partial phonon DOS calculation of the active region with a simple correction term accounting for the environment, under reasonable assumptions regarding atomic interactions. Convergence with respect to the size of the active region is investigated for different systems, as well as the reduction in computational costs when compared to full phonon DOS calculation. Additionally, we provide an open source implementation of the algorithm that can be added as an extension to LAMMPS software.

cond-mat.mtrl-sci↗

arXiv2017

A Tetrahedron-tiling Method for Crystal Structure Prediction

Reliable and robust methods of predicting the crystal structure of a compound, based only on its chemical composition, is crucial to the study of materials and their applications. Despite considerable ongoing research efforts, crystal structure prediction remains a challenging problem that demands large computational resources. Here we propose an efficient approach for first-principles crystal structure prediction. The new method explores and finds crystal structures by tiling together elementary tetrahedra that are energetically favorable and geometrically matching each other. This approach has three distinguishing features: a favorable building unit, an efficient calculation of local energy, and a stochastic Monte Carlo simulation of crystal growth. By applying the method to the crystal structure prediction of various materials, we demonstrate its validity and potential as a promising alternative to current methods.

cond-mat.mtrl-sci↗

arXiv2017

Free energy calculation of mechanically unstable but dynamically stabilized bcc titanium

The phase diagram of numerous materials of technological importance features high-symmetry high-temperature phases that exhibit phonon instabilities. Leading examples include shape-memory alloys, as well as ferroelectric, refractory, and structural materials. The thermodynamics of these phases have proven challenging to handle by atomistic computational thermodynamic techniques, due to the occurrence of constant anharmonicity-driven hopping between local low-symmetry distortions, while maintaining a high-symmetry time-averaged structure. To compute the free energy in such phases, we propose to explore the system's potential-energy surface by discrete sampling of local minima by means of a lattice gas Monte Carlo approach and by continuous sampling by means of a lattice dynamics approach in the vicinity of each local minimum. Given the proximity of the local minima, it is necessary to carefully partition phase space by using a Voronoi tessellation to constrain the domain of integration of the partition function, in order to avoid double-counting artifacts and enable an accurate harmonic treatment near each local minima. We consider the bcc phase of titanium as a prototypical examples to illustrate our approach.

cond-mat.mtrl-sci↗

arXiv2017

An epicycle method for elasticity limit calculations

The task of finding the smallest energy needed to bring a solid to its onset of mechanical instability arises in many problems in materials science, from the determination of the elasticity limit to the consistent assignment of free energies to mechanically unstable phases. However, unless the space of possible deformations is low-dimensional and a priori known, this problem is numerically difficult, as it involves minimizing a function under a constraint on its Hessian, which is computionally prohibitive to obtain in low symmetry systems, especially if electronic structure calculations are used. We propose a method that is inspired by the well-known dimer method for saddle point searches but that adds the necessary ingredients to solve for the lowest onset of mechanical instability. The method consists of two nested optimization problems. The inner one involves a dimer-like construction to find the direction of smallest curvature as well as the gradient of this curvature function. The outer optimization then minimizes energy using the result of the inner optimization problem to constrain the search to the hypersurface enclosing all points of zero minimum curvature. Example applications to both model systems and electronic structure calculations are given.

cond-mat.stat-mech↗

arXiv2014

Ab initio calculation of anisotropic interfacial excess free energies

We describe a simple method to determine, from ab initio calculations, the complete orientation-dependence of interfacial free energies in solid-state crystalline systems. We illustrate the method with an application to precipitates in the Al-Ti alloy system. The method combines the cluster expansion formalism in its most general form (to model the system's energetics) with the inversion of the well-known Wulff construction (to recover interfacial energies from equilibrium precipitate shapes). Although the inverse Wulff construction only provides the relative magnitude of the various interfacial free energies, absolute free energies can be recovered from a calculation of a single, conveniently chosen, planar interface. The method is able to account for essentially all sources of entropy (arising from phonons, bulk point defects, as well as interface roughness) and is thus able to transparently handle both atomically smooth and rough interfaces. The approach expresses the resulting orientation-dependence of the interfacial properties using symmetry-adapted bases for general orientation-dependent quantities. As a by-product, this paper thus provides a simple and general method to generate such basis functions, which prove useful in a variety of other applications, for instance to represent the anisotropy of the so-called constituent strain elastic energy.

cond-mat.mtrl-sci↗

arXiv2013

Realistic time-scale fully atomistic simulations of surface nucleation of dislocations in pristine nanopillars

We use our recently proposed accelerated dynamics algorithm (Tiwary & van de Walle, 2011) to calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we achieve the fraction of a second time-scale regime. We find that the activation free energy depends significantly on the driving force (stress or strain) and temperature, leading to very high activation entropies. We also perform compression tests on Gold nanopillars for strain rates varying between 7 orders of magnitudes, reaching as low as 10^3/s. Our calculations show the quantitative effects on the yield point of unrealistic strain-rate Molecular Dynamics calculations: we find that while the failure mechanism for <001> compression of Gold nanopillars remains the same across the entire strain-rate range, the elastic limit (defined as stress for nucleation of the first dislocation) depends significantly on the strain-rate. We also propose a new methodology that overcomes some of the limits in our original accelerated dynamics scheme (and accelerated dynamics methods in general). We lay out our methods in sufficient details so as to be used for understanding and predicting deformation mechanism under realistic driving forces for various problems.

cond-mat.mtrl-sci↗

arXiv2013

Accelerated Molecular Dynamics through stochastic iterations to strengthen yield of path hopping over upper states (SISYPHUS)

We present a new method, called SISYPHUS (Stochastic Iterations to Strengthen Yield of Path Hopping over Upper States), for extending accessible time-scales in atomistic simulations. The method proceeds by separating phase space into basins, and transition regions between the basins based on a general collective variable (CV) criterion. The transition regions are treated via traditional molecular dynamics (MD) while Monte Carlo (MC) methods are used to (i) estimate the expected time spent in each basin and (ii) thermalize the system between two MD episodes. In particular, an efficient adiabatic switching based scheme is used to estimate the time spent inside the basins. The method offers various advantages over existing approaches in terms of (i) providing an accurate real time scale, (ii) avoiding reliance on harmonic transition state theory and (iii) avoiding the need to enumerate all possible transition events. Applications of SISYPHUS to low temperature vacancy diffusion in BCC Ta and adatom island ripening in FCC Al are presented. A new CV appropriate for such condensed phases, especially for transitions involving collective motions of several atoms, is also introduced.

cond-mat.mtrl-sci↗

arXiv2012

Ab initio thermodynamics of intrinsic oxygen vacancies in ceria

Nonstoichiometric ceria(CeO$_{2-δ}$) is a candidate reaction medium to facilitate two step water splitting cycles and generate hydrogen. Improving upon its thermodynamic suitability through doping requires an understanding of its vacancy thermodynamics. Using density functional theory(DFT) calculations and a cluster expansion based Monte Carlo simulations, we have studied the high temperature thermodynamics of intrinsic oxygen vacancies in ceria. The DFT+$U$ approach was used to get the ground state energies of various vacancy configurations in ceria, which were subsequently fit to a cluster expansion Hamiltonian to efficiently model the configurational dependence of energy. The effect of lattice vibrations was incorporated through a temperature dependent cluster expansion. Lattice Monte Carlo simulations using the cluster expansion Hamiltonian were able to detect the miscibility gap in the phase diagram of ceria. The inclusion of vibrational and electronic entropy effects made the agreement with experiments quantitative. The deviation from an ideal solution model was quantified by calculating as a function of nonstoichiometry, a) the solid state entropy from Monte Carlo simulations and b) Warren-Cowley short range order parameters of various pair clusters.

cond-mat.mtrl-sci↗