arXiv subjects

David Prendergast

Publications and source records attributed to David Prendergast.

At least 19 recordsLinked to original sources

arXiv2026

Practical Considerations for Finite Concentrations Molecular Dynamics Simulations

Understanding concentrated electrolytes requires a theory that spans local hydration and mesoscale interfacial assembly. We present an integrated workflow-SCOPE-that combines (i) enhanced sampling focused on a single Li+ ion, (ii) reweighting of biased trajectories to recover equilibrium microstate probabilities, and (iii) a chemical-potential correction that accounts for the limited reservoir of free water in finite simulation boxes. Applied to LiCl(aq) across 0.5-26 M and 283-313 K, this approach reveals a simple organizing principle: solvated ions dominate at low concentration; contact ion pairs emerge at intermediate strength; and aggregated Li-xCl clusters become most stable at the solubility limit. The resulting free-energy trends predict temperature-dependent solubility in close agreement with experiment and clarify the role of interfacial nucleation in precipitation. Beyond the simple LiCl(aq) salt considered here, SCOPE offers a transferable strategy for characterizing speciation and phase behavior in concentrated liquid systems where collective coordinates and rare events dominate.

cond-mat.soft↗

arXiv2025

Deriving effective electrode-ion interactions from free-energy profiles at electrochemical interfaces

Understanding ion adsorption at electrified metal-electrolyte interfaces is essential for accurate modeling of electrochemical systems. Here, we systematically investigate the free energy profiles of Na$^+$, Cl$^-$, and F$^-$ ions at the Au(111)-water interface using enhanced sampling molecular dynamics with both classical force fields and machine-learned interatomic potentials (MLIPs). Our classical metadynamics results reveal a strong dependence of predicted ion adsorption on the Lennard-Jones parameters, highlighting that -- without due care -- standard mixing rules can lead to qualitatively incorrect descriptions of ion-metal interactions. We present a systematic methodology for tuning the cross-term LJ parameters to control adsorption energetics in agreement with more accurate models. As a surrogate for an ab initio model, we employed the recently released Universal Models for Atoms (UMA) MLIP, which validates classical trends and displays strong specific adsorption for chloride, weak adsorption for fluoride, and no specific adsorption for sodium, in agreement with experimental and theoretical expectations. By integrating molecular-level adsorption free energies into continuum models of the electric double layer, we show that specific ion adsorption substantially alters the interfacial ion population, the potential of zero charge, and the differential capacitance of the system. Our results underscore the critical importance of force field parameterization and advanced interatomic potentials for the predictive modeling of ion-specific effects at electrified interfaces and provide a robust framework for bridging molecular simulations and continuum electrochemical models.

physics.chem-ph↗

arXiv2024

Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH$_4$)$_2$ in THF

Divalent ions, such as Mg, Ca, and Zn, are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry. However, the challenge remains to match electrode and electrolyte materials that stably cycle with these new formulations, based primarily on controlling interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH$_4$)$_2$ in THF through molecular dynamics simulation. Free-energy analysis indicates that this electrolyte has a majority population of neutral Ca dimers and monomers, albeit with diverse molecular conformations as revealed by unsupervised learning techniques, but with an order of magnitude lower concentration of possibly electroactive charged species, such as the monocation, CaBH$_4^+$ , which we show is produced via disproportionation of neutral Ca(BH$_4$)$_2$ complexes. Dense layering of THF molecules within 1 nm of the electrode surface (modeled here using graphite) hinders the approach of reducible species to within 0.6 nm and instead enhances the local concetration of species in a narrow intermediate-density layer from 0.7-0.9 nm. A dramatic increase in the monocation population in this intermediate layer is induced at negative bias, supplied by local dimer disproportionation. We see no evidence to support any functional role of fully-solvated Ca$^{2+}$ in the electrochemical activity of this electrolyte. The consequences for performance and alternative formulations are discussed in light of this molecular-scale insight.

cond-mat.mtrl-sci↗

arXiv2023

Unsupervised learning of representative local atomic arrangements in molecular dynamics data

Molecular dynamics (MD) simulations present a data-mining challenge, given that they can generate a considerable amount of data but often rely on limited or biased human interpretation to examine their information content. By not asking the right questions of MD data we may miss critical information hidden within it. We combine dimensionality reduction (UMAP) and unsupervised hierarchical clustering (HDBSCAN) to quantitatively characterize prevalent coordination environments of chemical species within MD data. By focusing on local coordination, we significantly reduce the amount of data to be analyzed by extracting all distinct molecular formulas within a given coordination sphere. We then efficiently combine UMAP and HDBSCAN with alignment or shape-matching algorithms to partition these formulas into structural isomer families indicating their relative populations. The method was employed to reveal details of cation coordination in electrolytes based on molecular liquids.

cond-mat.mtrl-sci↗

arXiv2022

Investigating the Electronic Structure of Prospective Water-splitting Oxide BaCe$_{0.25}$Mn$_{0.75}$O$_{3-δ}$ Before and After Thermal Reduction

BaCe$_{0.25}$Mn$_{0.75}$O$_{3-δ}$ (BCM), a non-stoichiometric oxide closely resembling a perovskite crystal structure, has recently emerged as a prospective contender for application in renewable energy harvesting by solar thermochemical hydrogen generation. Using solar energy, oxygen-vacancies can be created in BCM and the reduced crystal so obtained can, in turn, produce H2 by stripping oxygen from H2O. Therefore, a first step toward understanding the working mechanism and optimizing the performance of BCM, is a thorough and comparative analysis of the electronic structure of the pristine and the reduced material. In this paper, we probe the electronic structure of BCM using the combined effort of first-principles calculations and experimental O K-edge x-ray absorption spectroscopy (XAS). The computed projected density-of-states (PDOS) and orbital-plots are used to propose a simplified model for orbital-mixing between the oxygen and the ligand atoms. With the help of state-of-the-art simulations, we are able to find the origins of the XAS peaks and to categorize them on the basis of contribution from Ce and Mn. For the reduced crystal, the calculations show that, as a consequence of dielectric screening, the change in electron-density resulting from the reduction is strongly localized around the oxygen vacancy. Our experimental studies reveal a marked lowering of the first O K-edge peak in the reduced crystal which is shown to result from a diminished O-2p contribution to the frontier unoccupied orbitals, in accordance with the tight-binding scheme. Our study paves the way for investigation of the working-mechanism of BCM and for computational and experimental efforts aimed at design and discovery of efficient water-splitting oxides.

cond-mat.mtrl-sci↗

arXiv2022

Excitonic effects in X-ray absorption spectra of fluoride salts and their surfaces

Operando X-ray absorption spectroscopy (XAS) can probe the electrode-electrolyte interface with single-digit nanometer depth resolution and offers a wealth of insight into the evolution and Coulombic efficiency or degradation of prototype cells, provided that the spectra can be reliably interpreted in terms of local oxidation state, atomic coordination, and electronic structure aboutthe excited atoms. To this end, we explore fluorine K-edge XAS of mono- (Li, Na, K) and di-valent (Mg, Ca, Zn) fluoride salts from a theoretical standpoint and discover a surprising level of detailed electronic structure information about these materials, despite the relatively predictable oxidation state and ionicity of the fluoride anion and the metal cation. Utilizing a recently developed many-body approach based on the delta-SCF method, we calculate the XAS using density functional theory and experimental spectral profiles are well reproduced, despite some experimental discrepancies in energy alignment within the literature, which we can correct for in our simulations. We outline a general methodology to explain shifts in the main XAS peak energies in terms of a simple exciton model and explain line-shape differences resulting from mixing of core-excited states with metal d character (for K and Ca specifically). Given ultimate applications to evolving interfaces, some understanding of the role of surfaces and their terminations in defining new spectral features is provided to indicate the sensitivity of such measurements to changes in interfacial chemistry.

cond-mat.mtrl-sci↗

arXiv2022

Efficient core-excited state orbital perspective on calculating X-ray absorption transitions in determinant framework

X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analysing such spectra are therefore in high demand for complementing the experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled Many-Body XAS (MBXAS), which approximates the final (initial) state as a Slater determinant constructed from Kohn-Sham (KS) orbitals optimized in absence (presence) of the relevant core-hole has shown promising prospects in evaluating the transition amplitudes. In this article, we show that the MBXAS approach can be rederived using a transition operator expressed entirely in the basis of core-excited state KS orbitals and that this reformulation offers substantial practical and conceptual advantages. In addition to circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals, the aforementioned representation reduces the computational expense by rendering the calculation of such orbitals unnecessary altogether. The reformulated approach also provides a direct pathway for comparing the many-body approximation with the so-called single-particle treatment and indicates the relative importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation. Finally, using the core-excited state basis, we define auxiliary orbitals for x-ray absorption and demonstrate their utility in explaining the spectral intensity by contrasting them with single-particle approximations to the excited state.

cond-mat.mtrl-sci↗

arXiv2022

CleaRIXS: A Fast and Accurate First-Principles Method for Simulation and Analysis of Resonant Inelastic X-ray Scattering

Resonant Inelastic X-ray Scattering (RIXS), which probes the occupied and unoccupied electronic subspaces in an interrelated fashion, is one of the most detailed, complex and information-rich experimental techniques employed in the investigation of electronic structure across physics, chemistry and materials science. We introduce an ab-initio, accurate and efficient computational framework for simulation and analysis of RIXS spectra, which combines two diverse and established approaches to modeling electronic excited states. The Core-Hole Linear-Response RIXS (CleaRIXS) method not only ensures accurate incorporation of the interaction of electrons with core and valence holes, but also automatically maps the salient RIXS features to the relevant electronic excitations and de-excitations. Through direct comparison with previous methanol C K-edge RIXS measurements [J. Phys. Chem. A 120, 2260 (2016)], we show the efficacy of the formalism in modeling different regions of the RIXS spectrum and in gaining physical insight regarding their origins. The importance of including the valence electron-hole interactions is explored, in addition to the connection between CleaRIXS and determinant-based approaches for simulating X-ray absorption and non-resonant X-ray emission. A qualitative discussion of additional RIXS features arising from possible vibrationally-mediated processes is also presented. CleaRIXS provides a robust and extendable framework for prediction and interpretation of RIXS processes and for the simulation of complex electronic excited states in general.

cond-mat.mtrl-sci↗

arXiv2022

Direct observation of enhanced electron-phonon coupling in copper nanoparticles in the warm-dense matter regime

Warm-dense matter (WDM) is a highly-excited state that lies at the confluence of solids, plasmas, and liquids and that cannot be described by equilibrium theories. The transient nature of this state when created in a laboratory, as well as the difficulties in probing the strongly-coupled interactions between the electrons and the ions, make it challenging to develop a complete understanding of matter in this regime. In this work, by exciting isolated ~8 nm nanoparticles with a femtosecond laser below the ablation threshold, we create uniformly-excited WDM. We then use photoelectron spectroscopy to track the instantaneous electron temperature and directly extract the strongest electron-ion coupling observed experimentally to date. By directly comparing with state-of-the-art theories, we confirm that the superheated nanoparticles lie at the boundary between hot solids and plasmas, with associated strong electron-ion coupling. This is evidenced both by the fast energy loss of electrons to ions, as well as a strong modulation of the electron temperature by acoustic oscillations in the nanoparticle. This work demonstrates a new route for experimental exploration and theoretical validation of the exotic properties of WDM.

physics.plasm-ph↗

arXiv2021

Changes in polarization dictate necessary approximations for modeling electronic de-excitation intensity: an application to X-ray emission

We systematically investigate the underlying relations among different levels of approximation for simulating electronic de-excitations, with a focus on modeling X-ray emission spectroscopy (XES). Using Fermi's golden rule and explicit modeling of the initial, core-excited state and the final, valence-hole state, we show that XES can be accurately modeled by using orbital optimization for the various final states within a Slater-determinant framework. However, in this paper, we introduce a much cheaper approach reliant only on a single self-consistent field for all the final states, and show that it is typically sufficient. Further approximations reveal that these fundamentally many-body transitions can be reasonably approximated by projections of ground state orbitals, but that the ground state alone is insufficient. Furthermore, except in cases where the core-ionization induces negligible changes in polarization, linear-response approaches within the adiabatic approximation will have difficulty in accurately modeling de-excitation to the core level. Therefore, change in the net dipole moment of the valence electrons can serve as a metric for the validity of the linear-response approximation.

cond-mat.mtrl-sci↗

arXiv2020

Harnessing Structural and Dynamic Heterogeneity to Direct Ion Transport in Plastic Crystal-Polymer Composite Solid-Ion Conductors

Solid-ion conductors (SICs) comprising non-ionic plastic crystals and lithium salts often require compositing with polymers to render them processable for use in solid-state lithium-metal batteries. Here, we show that polymer-doped plastic crystal SICs form a previously unrecognized plastic crystal-polymer high entropy interphase, where ions selectively partition and exhibit a higher fraction of matrix-separated ion pairs than in the bulk. Liithium ion diffusivity in this interphase is an order of magnitude higher than in other microenvironments due to an increase in the molar volume of the plastic crystal in the vicinity of the polymer, which increases the frequency of bond rotation in the plastic crystal required for ion conduction. Surprisingly, the polymer does not directly participate in ion transport. These insights led us to prepare SICs from specific polymers, plastic crystals, and lithium halide salts that concomitantly deliver fast ion conduction at ambient and sub-ambient temperatures and sustainable passivation of the lithium anode.

physics.app-ph↗

arXiv2020

Layer Edge States Stabilized by Internal Electric Fields in Two-dimensional Hybrid Perovskites

Two-dimensional (2D) organic-inorganic hybrid perovskites have been intensively explored for recent years, due to their tunable band gaps and exciton binding energies, and increased stability with respect to three-dimensional (3D) hybrid perovskites. There were fascinating experimental observations suggesting the existence of localized edge states in 2D hybrid perovskites which facilitate extremely efficient electron-hole dissociation and long carrier lifetimes. The observations and explanations of the edge states are not quite converging implying that there can be multiple origins for the edge state formation. Using first principles calculations, we demonstrate that layer edge states are stabilized by internal electric fields created by polarized molecular alignment of organic cations in 2D hybrid perovskites when they are two layers or thicker. Our study gives a simple physical explanation of the edge state formation, and it will pave the way for designing and manipulating layer edge states for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C$_{3}$H$_{7}$I and t-C$_{4}$H$_{9}$I) through a conical intersection between $^{3}$Q$_{0}$/$^{1}$Q$_{1}$ spin-orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the $^{3}$Q$_{0}$ state into the $^{1}$Q$_{1}$ state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C$_{3}$H$_{7}$I and t-C$_{4}$H$_{9}$I directly lead to differences in the photoproduct branching ratio of the two systems.

physics.chem-ph↗

arXiv2019

Diversity-Oriented Synthesis of Polymers of Intrinsic Microporosity with Explicit Solid Solvation Cages for Lithium Ions

Here, we describe a diversity-oriented synthetic strategy for microporous polymer membranes from which we identified those whose FVEs serve as solid solvation cages for lithium ions. Lead candidate membranes featuring such ion solvation cages exhibited both higher ionic conductivity and higher cation transference number than control membranes where FVEs were aspecific, which indicates conventional bounds for membrane permeability and selectivity for ion transport can be overcome. Such membranes show promise as dendrite-suppressing anode-electrolyte interlayers in high-voltage lithium-metal batteries for electric mobility.

cond-mat.mtrl-sci↗

arXiv2019

Taming Convergence in the Determinant Approach for X-Ray Excitation Spectra

A determinant formalism in combination with \emph{ab initio} calculations proposed recently has paved a new way for simulating and interpreting x-ray excitation spectra. The new method systematically takes into account many-electron effects in the Mahan-Noziéres-De Dominicis (MND) theory, including core-level excitonic effects, the Fermi-edge singularity, shakeup excitations, and wavefunction overlap effects such as the orthogonality catastrophe, all within a universal framework using many-electron configurations. A heuristic search algorithm was introduced to search for the configurations that are important for defining spectral lineshapes, instead of enumerating them in a brute-force way. The algorithm has proven to be efficient for calculating \ce{O} $K$ edges of transition metal oxides, which converge at the second excitation order (denoted as $f^{(n)}$ with $n = 2$), i.e., the final-state configurations with two \emph{e-h} pairs (with one hole being the core hole). However, it remains unknown how the determinant calculations converge for general cases and at which excitation order $n$ one should stop the determinant calculation. Even with the heuristic algorithm, the number of many-electron configurations still grows exponentially with the excitation order $n$. In this work, we prove two theorems that can indicate the order of magnitude of the contribution of the $f^{(n)}$ configurations, so that one can estimate their contribution very quickly without actually calculating their amplitudes. The two theorems are based on the singular-value decomposition (SVD) analysis, a method that is widely used to quantify entanglement between two quantum many-body systems. We examine the $K$ edges of several metallic systems with the determinant formalism up to $f^{(5)}$ to illustrate the usefulness of the theorems.

cond-mat.mtrl-sci↗

arXiv2018

Probing the electric field-induced doping mechanism in YBa2Cu3O7 using computed Cu K-edge x-ray absorption spectra

We recently demonstrated that the superconductor-to-insulator transition induced by ionic liquid gating of the high temperature superconductor YBa2Cu3O7 (YBCO) is accompanied by a deoxygenation of the sample [Perez-Munoz et al., PNAS 114, 215 (2017)]. DFT calculations helped establish that the pronounced changes in the spectral features of the Cu K-edge absorption spectra measured in situ during the gating experiment arise from a decrease of the Cu coordination within the CuO chains. In this work, we provide a detailed analysis of the electronic structure origin of the changes in the spectra resulting from three different types of doping: i) the formation of oxygen vacancies within the CuO chains, ii) the formation of oxygen vacancies within the CuO2 planes and iii) the electrostatic doping. For each case, three stoichiometries are studied and compared to the stoichiometric YBa2Cu3O7, i.e YBa2Cu3O6.75, YBa2Cu3O6.50 and YBa2Cu3O6.25. Computed vacancy formation energies further support the chain-vacancy mechanism. In the case of doping by vacancies within the chains, we study the effect of oxygen ordering on the spectral features and we clarify the connection between the polarization of the x-rays and this doping mechanism. Finally, the inclusion of the Hubbard U correction on the computed spectra for antiferromagnetic YBa2Cu3O6.25 is discussed.

cond-mat.mtrl-sci↗

arXiv2018

Quantum Many-Body Effects in X-Ray Spectra Efficiently Computed using a Basic Graph Algorithm

The growing interest in using x-ray spectroscopy for refined materials characterization calls for accurate electronic-structure theory to interpret x-ray near-edge fine structure. In this work, we propose an efficient and unified framework to describe all the many-electron processes in a Fermi liquid after a sudden perturbation (such as a core hole). This problem has been visited by the Mahan-Noziéres-De Dominicis (MND) theory, but it is intractable to implement various Feynman diagrams within first-principles calculations. Here, we adopt a non-diagrammatic approach and treat all the many-electron processes in the MND theory on an equal footing. Starting from a recently introduced determinant formalism [Phys. Rev. Lett. 118, 096402 (2017)], we exploit the linear-dependence of determinants describing different final states involved in the spectral calculations. An elementary graph algorithm, breadth-first search, can be used to quickly identify the important determinants for shaping the spectrum, which avoids the need to evaluate a great number of vanishingly small terms. This search algorithm is performed over the tree-structure of the many-body expansion, which mimics a path-finding process. We demonstrate that the determinantal approach is computationally inexpensive even for obtaining x-ray spectra of extended systems. Using Kohn-Sham orbitals from two self-consistent fields (ground and core-excited state) as input for constructing the determinants, the calculated x-ray spectra for a number of transition metal oxides are in good agreement with experiments. Many-electron aspects beyond the Bethe-Salpeter equation, as captured by this approach, are also discussed, such as shakeup excitations and many-body wave function overlap considered in Anderson's orthogonality catastrophe.

cond-mat.mtrl-sci↗

arXiv2017

Cooperative gas adsorption without a phase transition in metal-organic frameworks

Cooperative adsorption of gases by porous frameworks permits more efficient uptake and removal than does the more usual non-cooperative (Langmuir-type) adsorption. Cooperativity, signaled by a step-like isotherm, is usually attributed to a phase transition of the framework. However, the class of metal-organic frameworks mmen-M$_2$(dobpdc) exhibit cooperative adsorption of CO2 but show no evidence of a phase transition. Here we show how cooperativity emerges in these frameworks in the absence of a phase transition. We use a combination of quantum and statistical mechanics to show that cooperativity results from a sharp but finite increase, with pressure, of the mean length of chains of CO2 molecules that polymerize within the framework. Our study provides microscopic understanding of the emergent features of cooperative binding, including the position, slope and height of the isotherm step, and indicates how to optimize gas storage and separation in these materials.

cond-mat.mtrl-sci↗