arXiv subjects

Xavier Andrade

Publications and source records attributed to Xavier Andrade.

At least 19 recordsLinked to original sources

arXiv2025

SHarmonic: A fast and accurate implementation of spherical harmonics for electronic-structure calculations

The authors present SHarmonic, a new implementation of the spherical harmonics targeted for electronic-structure calculations. Their approach is to use explicit formulas for the harmonics written in terms of normalized Cartesian coordinates. This approach results in a code that is as precise as other implementations while being at least one order of magnitude more computationally efficient. The library can run on graphics processing units (GPUs) as well, achieving an additional order of magnitude in execution speed. This new implementation is simple to use and is provided under an open source license, it can be readily used by other codes to avoid the error-prone and cumbersome implementation of the spherical harmonics.

physics.comp-ph↗

arXiv2025

Spin non-Collinear Real-Time Time-Dependent Density-Functional Theory and Implementation in the Modern GPU-Accelerated INQ code

Time-dependent density functional theory (TDDFT) is a theory that describes the time evolution of quantum mechanical many-electron systems under the influence of external time-dependent electric and magnetic fields. INQ is a specially designed software to efficiently solve the real-time TDDFT equations on graphics processing units (GPUs), which aim to overcome the computational limitation of time and size scales of non-equilibrium quantum dynamics. In this work we will present an implementation of non-collinear TDDFT for the INQ code to simulate spin dynamics in real time and discuss the implementation of non-collinear magnetic effects into the code. We will discuss the implementation of exchange-correlation magnetic fields, spin-orbit coupling, and the interaction between the electronic system and external magnetic fields. We will then consider several prototypical examples of spin dynamics in magnetic clusters and solids after light excitation. Potential applications range from the study of real-time dynamics of magnons to ultrafast spin dynamics under linear and circularly polarized laser excitation, as well as spectroscopic signatures such as magnetic circular dichroism and pump-probe Kerr rotation.

cond-mat.mtrl-sci↗

arXiv2025

Anomalies in the electronic stopping of slow antiprotons in LiF

We present first-principles theoretical calculations for the electronic stopping power (SP) of both protons and anti-protons in LiF. Our results show the presence of the Barkas effect: a higher stopping for positively charged particles than their negatively charged antiparticles. In contrast, a previous study has predicted an anti-Barkas effect (higher stopping for negative charges) at low velocity [Qi, Bruneval and Maliyov, Phys. Rev. Lett. 128, 043401 (2022)]. We explain this discrepancy by showing that this anti-Barkas effect appears for highly symmetric trajectories and disappears when considering trajectories that better reproduce the experimental setup. Our low-velocity results show that the SP of both protons and anti-proton vanish for velocities under 0.1 a.u. .

cond-mat.other↗

arXiv2024

Dynamics of ballistic photocurrents driven by Coulomb scattering

First principles real-time time dependent density functional theory (rt-TDDFT) calculations reveal the existence of ballistic photocurrents generated by Coulomb scattering, which has not previously been considered as a mechanism for the bulk photovoltaic effect. With monolayer GeS as an example, it is predicted that ballistic currents can be comparable to shift currents under experimentally accessible conditions.

cond-mat.mes-hall↗

arXiv2024

Multi-Objective Optimization of Consumer Group Autoscaling in Message Broker Systems

Message brokers often mediate communication between data producers and consumers by adding variable-sized messages to ordered distributed queues. Our goal is to determine the number of consumers and consumer-partition assignments needed to ensure that the rate of data consumption keeps up with the rate of data production. We model the problem as a variable item size bin packing problem. As the rate of production varies, new consumer-partition assignments are computed, which may require rebalancing a partition from one consumer to another. While rebalancing a queue, the data being produced into the queue is not read leading to additional latency costs. As such, we focus on the multi-objective optimization cost of minimizing both the number of consumers and queue migrations. We present a variety of algorithms and compare them to established bin packing heuristics for this application. Comparing our proposed consumer group assignment strategy with Kafka's, a commonly employed strategy, our strategy presents a 90th percentile latency of 4.52s compared to Kafka's 217s with both using the same amount of consumers. Kafka's assignment strategy only improved the consumer group's performance with regards to latency with configurations that used at least 60% more resources than our approach.

cs.DC↗

arXiv2022

Kafka Consumer Group Autoscaler

Message brokers enable asynchronous communication between data producers and consumers in distributed environments by assigning messages to ordered queues. Message broker systems often provide with mechanisms to parallelize tasks between consumers to increase the rate at which data is consumed. The consumption rate must exceed the production rate or queues would grow indefinitely. Still, consumers are costly and their number should be minimized. We model the problem of determining the required number of consumers, and the partition-consumer assignments, as a variable item size bin packing variant. Data cannot be read when a queue is being migrated to another consumer. Hence, we propose the R-score metric to account for these rebalancing costs. Then, we introduce an assortment of R-score based algorithms, and compare their performance to established heuristics for the Bin Packing Problem for this application. We instantiate our method within an existing system, demonstrating its effectiveness. Our approach guarantees adequate consumption rates something the previous system was unable to at lower operational costs.

cs.DC↗

arXiv2022

Electron dynamics in extended systems within real-time time-dependent density functional theory

Due to a beneficial balance of computational cost and accuracy, real-time time-dependent density functional theory has emerged as a promising first-principles framework to describe electron real-time dynamics. Here we discuss recent implementations around this approach, in particular in the context of complex, extended systems. Results include an analysis of the computational cost associated with numerical propagation and when using absorbing boundary conditions. We extensively explore the shortcomings for describing electron-electron scattering in real time and compare to many-body perturbation theory. Modern improvements of the description of exchange and correlation are reviewed. In this work, we specifically focus on the Qb@ll code, which we have mainly used for these types of simulations over the last years, and we conclude by pointing to further progress needed going forward.

cond-mat.mtrl-sci↗

arXiv2021

INQ, a modern GPU-accelerated computational framework for (time-dependent) density functional theory

We present INQ, a new implementation of density functional theory (DFT) and time-dependent DFT (TDDFT) written from scratch to work on graphical processing units (GPUs). Besides GPU support, INQ makes use of modern code design features and takes advantage of newly available hardware. By designing the code around algorithms, rather than against specific implementations and numerical libraries, we aim to provide a concise and modular code. The result is a fairly complete DFT/TDDFT implementation in roughly 12,000 lines of open-source C++ code representing a modular platform for community-driven application development on emerging high-performance computing architectures for the simulation of materials.

cond-mat.mtrl-sci↗

arXiv2019

Octopus, a computational framework for exploring light-driven phenomena and quantum dynamics in extended and finite systems

Over the last years extraordinary advances in experimental and theoretical tools have allowed us to monitor and control matter at short time and atomic scales with a high-degree of precision. An appealing and challenging route towards engineering materials with tailored properties is to find ways to design or selectively manipulate materials, especially at the quantum level. To this end, having a state-of-the-art ab initio computer simulation tool that enables a reliable and accurate simulation of light-induced changes in the physical and chemical properties of complex systems is of utmost importance. The first principles real-space-based Octopus project was born with that idea in mind, providing an unique framework allowing to describe non-equilibrium phenomena in molecular complexes, low dimensional materials, and extended systems by accounting for electronic, ionic, and photon quantum mechanical effects within a generalized time-dependent density functional theory framework. The present article aims to present the new features that have been implemented over the last few years, including technical developments related to performance and massive parallelism. We also describe the major theoretical developments to address ultrafast light-driven processes, like the new theoretical framework of quantum electrodynamics density-functional formalism (QEDFT) for the description of novel light-matter hybrid states. Those advances, and other being released soon as part of the Octopus package, will enable the scientific community to simulate and characterize spatial and time-resolved spectroscopies, ultrafast phenomena in molecules and materials, and new emergent states of matter (QED-materials).

physics.comp-ph↗

arXiv2019

Effect of chemical disorder on the electronic stopping of solid solution alloys

The electronic stopping power of nickel-based equiatomic solid solutions alloys NiCr, NiFe and NiCo for protons and alpha projectiles is investigated in detail using real-time time-dependent density functional theory over a wide range of velocities. Recently developed numerical electronic structure methods are used to probe fundamental aspects of electron-ion coupling non-perturbatively and in a fully atomistic context, capturing the effect of the atomic scale disorder. The effects of particular electronic band structures and density of states reflect in the low velocity limit behavior. We compare our results for the alloys with those of a pure nickel target to understand how alloying affects the electronic stopping. We discover that NiCo and NiFe have similar stopping behavior as Ni while NiCr has an asymptotic stopping power that is more than a factor of two larger than its counterparts for velocities below 0.1 a.u.. We show that the low-velocity limit of electronic stopping power can be manipulated by controlling the broadening of the d-band through the chemical disorder. In this regime, the Bragg's additive rule for the stopping of composite materials also fails for NiCr.

cond-mat.mtrl-sci↗

arXiv2017

Self-interaction effects on charge-transfer collisions

In this article, we investigate the role of the self-interaction error in the simulation of collisions using time-dependent density functional theory (TDDFT) and Ehrenfest dynamics. We compare many different approximations of the exchange and correlation potential, using as a test system the collision of $\mathrm{H^+ + CH_4}$ at $30~\mathrm{eV}$. We find that semi-local approximations, like PBE, and even hybrid functionals, like B3LYP, produce qualitatively incorrect predictions for the scattering of the proton. This discrepancy appears because the self-interaction error allows the electrons to jump too easily to the proton, leading to radically different forces with respect to the non-self-interacting case. From our results, we conclude that using a functional that is self-interaction free is essential to properly describe charge-transfer collisions between ions and molecules in TDDFT.

physics.chem-ph↗

arXiv2017

Non-linear conductivity of metals from real-time quantum simulations

We simulate bulk materials under strong currents by following in real-time the dynamics of the electrons under an electric field. By changing the intensity of the electric field, our method can model, for the first time, non-linear effects in the conductivity from first principles. To illustrate our approach, we show calculations that predict that liquid aluminum exhibits negative-differential conductivity for current densities of the order of $10^{12}-10^{13}~\mathrm{A/cm^2}$. We find that the change in the non-linear conductivity emerges from a competition between the accumulation of charge around the nuclei that increases the scattering of the conduction electrons, and a decreasing scattering cross-section at high currents.

cond-mat.mtrl-sci↗

arXiv2015

Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems

Real-space grids are a powerful alternative for the simulation of electronic systems. One of the main advantages of the approach is the flexibility and simplicity of working directly in real space where the different fields are discretized on a grid, combined with competitive numerical performance and great potential for parallelization. These properties constitute a great advantage at the time of implementing and testing new physical models. Based on our experience with the Octopus code, in this article we discuss how the real-space approach has allowed for the recent development of new ideas for the simulation of electronic systems. Among these applications are approaches to calculate response properties, modeling of photoemission, optimal control of quantum systems, simulation of plasmonic systems, and the exact solution of the Schrödinger equation for low-dimensionality systems.

physics.chem-ph↗

arXiv2014

A sparse-sampling approach for the fast computation of matrices: application to molecular vibrations

This article presents a new method to compute matrices from numerical simulations based on the ideas of sparse sampling and compressed sensing. The method is useful for problems where the determination of the entries of a matrix constitutes the computational bottleneck. We apply this new method to an important problem in computational chemistry: the determination of molecular vibrations from electronic structure calculations, where our results show that the overall scaling of the procedure can be improved in some cases. Moreover, our method provides a general framework for bootstrapping cheap low-accuracy calculations in order to reduce the required number of expensive high-accuracy calculations, resulting in a significant 3x speed-up in actual calculations.

physics.chem-ph↗

arXiv2013

Real-space density functional theory on graphical processing units: computational approach and comparison to Gaussian basis set methods

We discuss the application of graphical processing units (GPUs) to accelerate real-space density functional theory (DFT) calculations. To make our implementation efficient, we have developed a scheme to expose the data parallelism available in the DFT approach; this is applied to the different procedures required for a real-space DFT calculation. We present results for current-generation GPUs from AMD and Nvidia, which show that our scheme, implemented in the free code Octopus, can reach a sustained performance of up to 90 GFlops for a single GPU, representing a significant speed-up when compared to the CPU version of the code. Moreover, for some systems our implementation can outperform a GPU Gaussian basis set code, showing that the real-space approach is a competitive alternative for DFT simulations on GPUs.

physics.comp-ph↗

arXiv2013

More accurate and efficient bath spectral densities from super-resolution

Quantum transport and other phenomena are typically modeled by coupling the system of interest to an environment, or bath, held at thermal equilibrium. Realistic bath models are at least as challenging to construct as models for the quantum systems themselves, since they must incorporate many degrees of freedom that interact with the system on a wide range of timescales. Owing to computational limitations, the environment is often modeled with simple functional forms, with a few parameters fit to experiment to yield semi-quantitative results. Growing computational resources have enabled the construction of more realistic bath models from molecular dynamics (MD) simulations. In this paper, we develop a numerical technique to construct these atomistic bath models with better accuracy and decreased cost. We apply a novel signal processing technique, known as super-resolution, combined with a dictionary of physically-motivated bath modes to derive spectral densities from MD simulations. Our approach reduces the required simulation time and provides a more accurate spectral density than can be obtained via standard Fourier transform methods. Moreover, the spectral density is provided as a convenient closed-form expression which yields an analytic time-dependent bath kernel. Exciton dynamics of the Fenna-Matthews-Olsen light-harvesting complex are simulated with a second order time-convolutionless master equation, and spectral densities constructed via super-resolution are shown to reproduce the dynamics using only a quarter of the amount of MD data.

quant-ph↗

arXiv2012

A survey of the parallel performance and the accuracy of Poisson solvers for electronic structure calculations

We present an analysis of different methods to calculate the classical electrostatic Hartree potential created by charge distributions. Our goal is to provide the reader with an estimation on the performance ---in terms of both numerical complexity and accuracy--- of popular Poisson solvers, and to give an intuitive idea on the way these solvers operate. Highly parallelisable routines have been implemented in the first-principle simulation code Octopus to be used in our tests, so that reliable conclusions about the capability of methods to tackle large systems in cluster computing can be obtained from our work.

physics.comp-ph↗

arXiv2011

Prediction of the derivative discontinuity in density functional theory from an electrostatic description of the exchange and correlation potential

We propose a new approach to approximate the exchange and correlation (XC) functional in density functional theory. The XC potential is considered as an electrostatic potential, generated by a fictitious XC density, which is in turn a functional of the electronic density. We apply the approach to develop a correction scheme that fixes the asymptotic behavior of any approximated XC potential for finite systems. Additionally, the correction procedure gives the value of the derivative discontinuity; therefore it can directly predict the fundamental gap as a ground-state property.

physics.chem-ph↗