arXiv subjects

Mariana Rossi

Publications and source records attributed to Mariana Rossi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Hexagonal ice density dependence on inter atomic distance changes due to nuclear quantum effects

Hexagonal ice ($\rm{I_h}$), the most common structure of ice, displays a variety of fascinating properties. Despite major efforts, a theoretical description of all its properties is still lacking. In particular, correctly accounting for its density and interatomic interactions is of utmost importance as a stepping stone for a deeper understanding of other properties. Deep potentials are a recent alternative to investigate the properties of {\iceIH}, which aims to match the accuracy of \textit{ab initio} simulations with the simplicity and scalability of classical molecular dynamics. This becomes particularly significant if one wishes to address nuclear quantum effects. In this work, we use machine learning potentials obtained for different exchange and correlation functionals to simulate the structural and vibrational properties of {\iceIH}. We show that most functionals overestimate the density of ice compared to experimental results. Furthermore, a quantum treatment of the nuclei leads to even further distancing from experiments. We understand this by highlighting how different inter-atomic interactions play a role in obtaining the equilibrium density. In particular, different from water clusters and bulk water, nuclear quantum effects lead to stronger H-bonds in {\iceIH}.

cond-mat.mtrl-sci↗

arXiv2025

Non-Markovian Effects in Quantum Rate Calculations of Hydrogen Diffusion with Electronic Friction

We address the challenge of incorporating non-Markovian electronic friction effects in quantum-mechanical approximations of dynamical observables. A generalized Langevin equation (GLE) is formulated for ring-polymer molecular dynamics (RPMD) rate calculations, which combines electronic friction with a description of nuclear quantum effects (NQEs) for adsorbates on metal surfaces. An efficient propagation algorithm is introduced that captures both the spatial dependence of friction strength and non-Markovian frictional memory. This framework is applied to a model of hydrogen diffusing on Cu(111) derived from ab initio density functional theory (DFT) calculations, revealing significant alterations in rate constants and tunnelling crossover temperatures due to non-Markovian effects. Our findings explain why previous classical molecular dynamics simulations with Markovian friction showed unexpectedly good agreement with experiment, highlighting the critical role of non-Markovian effects in first-principles atomistic simulations.

cond-mat.mtrl-sci↗

arXiv2025

Learning the Electrostatic Response of the Electron Density through a Symmetry-Adapted Vector Field Model

A current challenge in atomistic machine learning is that of efficiently predicting the response of the electron density under electric fields. We address this challenge with symmetry-adapted kernel functions that are specifically derived to account for the rotational symmetry of a three-dimensional vector field. We demonstrate the equivariance of the method on a set of rotated water molecules and show its high efficiency with respect to number of training configurations and features for liquid water and naphthalene crystals. We conclude showcasing applications for relaxed configurations of gold nanoparticles, reproducing the scaling law of the electronic polarizability with size, up to systems with more than 2000 atoms. By deriving a natural extension to equivariant learning models of the electron density, our method provides an accurate and inexpensive strategy to predict the electrostatic response of molecules and materials.

cond-mat.mtrl-sci↗

arXiv2025

Density-Functional Perturbation Theory with Numeric Atom-Centered Orbitals

This paper represents one contribution to a larger Roadmap article reviewing the current status of the FHI-aims code. In this contribution, the implementation of density-functional perturbation theory in a numerical atom-centered framework is summarized. Guidelines on usage and links to tutorials are provided.

cond-mat.mtrl-sci↗

arXiv2025

Polarisation, Born Effective Charges, and Topological Invariants via a Berry-Phase Approach

This paper represents one contribution to a larger Roadmap article reviewing the current status of the FHI-aims code. In this contribution, the implementation of polarization, Born-effective charges and topological invariants using a Berry-phase approach in a all-electron, numeric atom-centered orbitals framework is summarized. Guidelines on usage and links to tutorials are provided.

cond-mat.mtrl-sci↗

arXiv2024

Frontier orbitals control dynamical disorder in molecular semiconductors

Charge transport in organic semiconductors is limited by dynamical disorder. Design rules for new high-mobility materials have therefore focused on limiting its two foundations: structural fluctuations and the transfer integral gradient. However, it has remained unclear how these goals should be translated into molecular structures. Here we show that a specific shape of the frontier orbital, with a lack of nodes along the long molecular axis, reduces the transfer integral gradient and therefore the dynamical disorder. We investigated single crystals of the prototypical molecular semiconductors pentacene and picene by angle-resolved photoemission spectroscopy and dynamical disorder calculations. We found that picene exhibits a remarkably low dynamical disorder. By separating in- and out-of-plane components of dynamical disorder, we identify the reason as a reduced out-of-plane disorder from a small transfer integral derivative. Our results demonstrate that molecules with an armchair $π$-electron topology and same-phase frontier orbitals like picene are promising molecular building blocks for the next generation of organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2024

i-PI 3.0: a flexible and efficient framework for advanced atomistic simulations

Atomic-scale simulations have progressed tremendously over the past decade, largely due to the availability of machine-learning interatomic potentials. These potentials combine the accuracy of electronic structure calculations with the ability to reach extensive length and time scales. The i-PI package facilitates integrating the latest developments in this field with advanced modeling techniques, thanks to a modular software architecture based on inter-process communication through a socket interface. The choice of Python for implementation facilitates rapid prototyping but can add computational overhead. In this new release, we carefully benchmarked and optimized i-PI for several common simulation scenarios, making such overhead negligible when i-PI is used to model systems up to tens of thousands of atoms using widely adopted machine learning interatomic potentials, such as Behler-Parinello, DeePMD and MACE neural networks. We also present the implementation of several new features, including an efficient algorithm to model bosonic and fermionic exchange, a framework for uncertainty quantification to be used in conjunction with machine-learning potentials, a communication infrastructure that allows deeper integration with electronic-driven simulations, and an approach to simulate coupled photon-nuclear dynamics in optical or plasmonic cavities.

physics.chem-ph↗

arXiv2024

Quantum rates in dissipative systems with spatially varying friction

We investigate whether making the friction spatially dependent on the reaction coordinate introduces quantum effects into the thermal reaction rates for dissipative reactions. Quantum rates are calculated using the numerically exact multi-configuration time-dependent Hartree (MCTDH) method, as well as the approximate ring-polymer molecular dynamics (RPMD), ring-polymer instanton (RPI) methods, and classical mechanics. By conducting simulations across a wide range of temperatures and friction strengths, we can identify the various regimes that govern the reactive dynamics. At high temperatures, in addition to the spatial-diffusion and energy-diffusion regimes predicted by Kramer's rate theory, a (coherent) tunnelling-dominated regime is identified at low friction. At low temperatures, incoherent tunnelling dominates most of Kramer's curve, except at very low friction when coherent tunnelling becomes dominant. Unlike in classical mechanics, the bath's influence changes the equilibrium time-independent properties of the system, leading to a complex interplay between spatially dependent friction and nuclear quantum effects even at high temperatures. More specifically, a realistic friction profile can lead to an increase (decrease) of the quantum (classical) rates with friction within the spatial-diffusion regime, showing that classical and quantum rates display qualitatively different behaviours. Except at very low frictions, we find that RPMD captures most of the quantum effects in the thermal reaction rates.

physics.chem-ph↗

arXiv2024

Roadmap on Data-Centric Materials Science

Science is and always has been based on data, but the terms "data-centric" and the "4th paradigm of" materials research indicate a radical change in how information is retrieved, handled and research is performed. It signifies a transformative shift towards managing vast data collections, digital repositories, and innovative data analytics methods. The integration of Artificial Intelligence (AI) and its subset Machine Learning (ML), has become pivotal in addressing all these challenges. This Roadmap on Data-Centric Materials Science explores fundamental concepts and methodologies, illustrating diverse applications in electronic-structure theory, soft matter theory, microstructure research, and experimental techniques like photoemission, atom probe tomography, and electron microscopy. While the roadmap delves into specific areas within the broad interdisciplinary field of materials science, the provided examples elucidate key concepts applicable to a wider range of topics. The discussed instances offer insights into addressing the multifaceted challenges encountered in contemporary materials research.

cond-mat.mtrl-sci↗

arXiv2024

An atomic-scale perspective on individual thiol-terminated molecules anchored to single S vacancies in MoS$_2$

Sulphur vacancies in MoS$_2$ on Au(111) have been shown to be negatively charged as reflected by a Kondo resonance. Here, we use scanning tunneling microscopy to show that these vacancies serve as anchoring sites for thiol-based molecules (CF$_3$-3P-SH) with two distinct reaction products, one of them showing a Kondo resonance. Based on comparisons with density-functional theory (DFT) calculations, including a random structure search and computation of energies and electronic properties at a hybrid exchange-correlation functional level, we conclude that both anchored molecules are charge neutral. One of them is an anchored intact CF$_3$-3P-SH molecule while the other one is the result of catalytically activated dehydrogenation to CF$_3$-3P-S with subsequent anchoring. Our investigations highlight a perspective of functionalizing defects with thiol-terminated molecules that can be equipped with additional functional groups, such as charge donor- or acceptor-moieties, switching units or magnetic centers.

cond-mat.mtrl-sci↗

arXiv2024

Efficient All-electron Hybrid Density Functionals for Atomistic Simulations Beyond 10,000 Atoms

Hybrid density functional approximations (DFAs) offer compelling accuracy for ab initio electronic-structure simulations of molecules, nanosystems, and bulk materials, addressing some deficiencies of computationally cheaper, frequently used semilocal DFAs. However, the computational bottleneck of hybrid DFAs is the evaluation of the non-local exact exchange contribution, which is the limiting factor for the application of the method for large-scale simulations. In this work, we present a drastically optimized resolution-of-identity-based real-space implementation of the exact exchange evaluation for both non-periodic and periodic boundary conditions in the all-electron code FHI-aims, targeting high-performance CPU compute clusters. The introduction of several new refined Message Passing Interface (MPI) parallelization layers and shared memory arrays according to the MPI-3 standard were the key components of the optimization. We demonstrate significant improvements of memory and performance efficiency, scalability, and workload distribution, extending the reach of hybrid DFAs to simulation sizes beyond ten thousand atoms. As a necessary byproduct of this work, other code parts in FHI-aims have been optimized as well, e.g., the computation of the Hartree potential and the evaluation of the force and stress components. We benchmark the performance and scaling of the hybrid DFA based simulations for a broad range of chemical systems, including hybrid organic-inorganic perovskites, organic crystals and ice crystals with up to 30,576 atoms (101,920 electrons described by 244,608 basis functions).

cond-mat.mtrl-sci↗

arXiv2024

Ab initio electron-lattice downfolding: potential energy landscapes, anharmonicity, and molecular dynamics in charge density wave materials

The interplay of electronic and nuclear degrees of freedom presents an outstanding problem in condensed matter physics and chemistry. Computational challenges arise especially for large systems, long time scales, in nonequilibrium, or in systems with strong correlations. In this work, we show how downfolding approaches facilitate complexity reduction on the electronic side and thereby boost the simulation of electronic properties and nuclear motion - in particular molecular dynamics (MD) simulations. Three different downfolding strategies based on constraining, unscreening, and combinations thereof are benchmarked against full density functional calculations for selected charge density wave (CDW) systems, namely 1H-TaS$_2$, 1T-TiSe$_2$, 1H-NbS$_2$, and a one-dimensional carbon chain. We find that the downfolded models can reproduce potential energy surfaces on supercells accurately and facilitate computational speedup in MD simulations by about five orders of magnitude in comparison to purely ab initio calculations. For monolayer 1H-TaS$_2$ we report classical replica exchange and quantum path integral MD simulations, revealing the impact of thermal and quantum fluctuations on the CDW transition.

cond-mat.str-el↗

arXiv2023

Impact of nuclear effects on the ultrafast dynamics of an organic/inorganic mixed-dimensional interface

Electron dynamics at weakly bound interfaces of organic/inorganic materials are easily influenced by large-amplitude nuclear motion. In this work, we investigate the effects of different approximations to the equilibrium nuclear distributions on the ultrafast charge-carrier dynamics of a laser-excited hybrid organic/inorganic interface. By considering a prototypical system consisting of pyrene physisorbed on a MoSe$_2$ monolayer, we analyze linear absorption spectra, electronic density currents, and charge-transfer dynamics induced by a femtosecond pulse in resonance with the frontier-orbital transition in the molecule. The calculations are based on \textit{ab initio} molecular dynamics with classical and quantum thermostats, followed by time-dependent density-functional theory coupled to multi-trajectory Ehrenfest dynamics. We impinge the system with a femtosecond (fs) pulse of a few hundred GW/cm$^2$ intensity and propagate it for 100 fs. We find that the optical spectrum is insensitive to different nuclear distributions in the energy range dominated by the excitations localized on the monolayer. The pyrene resonance, in contrast, shows a small blue shift at finite temperatures, hinting at an electron-phonon-induced vibrational-level renormalization. The electronic current density following the excitation is affected by classical and quantum nuclear sampling through suppression of beating patterns and faster decay times. Interestingly, finite temperature leads to a longer stability of the ultrafast charge transfer after excitation. Overall, the results show that the ultrafast charge-carrier dynamics are dominated by electronic rather than by nuclear effects at the field strengths and time scales considered in this work.

cond-mat.mtrl-sci↗

arXiv2023

A Hybrid-DFT Study of Intrinsic Point Defects in $MX_2$ ($M$=Mo, W; $X$=S, Se) Monolayers

Defects can strongly influence the electronic, optical and mechanical properties of 2D materials, making defect stability under different thermodynamic conditions crucial for material-property engineering. In this paper, we present an account of the structural and electronic characteristics of point defects in monolayer transition metal dichalcogenides $MX_2$ with $M$=Mo/W and $X$= S/Se, calculated with density-functional theory using the hybrid HSE06 exchange correlation functional including many-body dispersion corrections. For the simulation of charged defects, we employ a charge compensation scheme based on the virtual crystal approximation (VCA). We relate the stability and the electronic structure of charged vacancy defects in monolayer MoS$_2$ to an explicit calculation of the S monovacancy in MoS$_2$ supported on Au(111), and find convincing indication that the defect is negatively charged. Moreover, we show that the finite-temperature vibrational contributions to the free energy of defect formation can change the stability transition between adatoms and monovacancies by 300--400 K. Finally, we probe defect vibrational properties by calculating a tip-enhanced Raman scattering image of a vibrational mode of a MoS$_2$ cluster with and without an S monovacancy.

cond-mat.mtrl-sci↗

arXiv2023

Predicting the Electronic Density Response of Condensed-Phase Systems to Electric Field Perturbations

We present a local and transferable machine learning approach capable of predicting the real-space density response of both molecules and periodic systems to external homogeneous electric fields. The new method, SALTER, builds on the Symmetry-Adapted Gaussian Process Regression SALTED framework. SALTER requires only a small, but necessary, modification to the descriptors used to represent the atomic environments. We present the performance of the method on isolated water molecules, bulk water and a naphthalene crystal. Root mean square errors of the predicted density response lie at or below 10% with barely more than 100 training structures. Derived quantities, such as polarizability tensors and even Raman spectra further derived from these tensors show a good agreement with those calculated directly from quantum mechanical methods. Therefore, SALTER shows excellent performance when predicting derived quantities, while retaining all of the information contained in the full electronic response. This method is thus capable of learning vector fields in a chemical context and serves as a landmark for further developments.

physics.chem-ph↗

arXiv2023

Ab initio study of water dissociation on a charged Pd(111) surface

Interactions between molecules and electrode surfaces play a key role in electrochemical processes and are a subject of extensive research, both experimental and theoretical. In this manuscript, we address the water dissociation reaction on a Pd(111) electrode surface, modelled as a slab embedded in an external electric field. We aim at unraveling the relationship between surface charge and zero-point-energy in aiding or hindering this reaction. We calculate energy barriers with dispersion-corrected density-functional theory and an efficient parallel implementation of the nudged-elastic-band method. We show that the lowest dissociation barrier, and consequently highest reaction rate, takes place when the field reaches a strength where two different geometries of the water molecule in the reactant state are equally stable. Zero-point energy contributions to this reaction, on the other hand, remain nearly constant across a wide range of electric field strengths, despite significant changes in the reactant state. Interestingly, we show that the application of electric fields that induce a negative charge on the surface can make nuclear tunneling more significant for these reactions.

physics.chem-ph↗

arXiv2023

Simulating Nuclear Dynamics with Quantum Effects

This paper, part of a Roadmap article, provides an account of the status and the current challenges in the area of nuclear quantum dynamics simulations, and presents advances in theory and computational techniques to address these challenges.

physics.chem-ph↗

arXiv2022

Roadmap on Electronic Structure Codes in the Exascale Era

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing.

cond-mat.mtrl-sci↗