Imperial College London

Dr Laura Ratcliff

Faculty of EngineeringDepartment of Materials

Honorary Lecturer
 
 
 
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Contact

 

laura.ratcliff08

 
 
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Location

 

2M14Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

40 results found

Kalha C, Ratcliff LE, Colombi G, Schlueter C, Dam B, Gloskovskii A, Lee T-L, Thakur PK, Bhatt P, Zhu Y, Osterwalder J, Offi F, Panaccione G, Regoutz Aet al., 2024, Revealing the Bonding Nature and Electronic Structure of Early-Transition-Metal Dihydrides, PRX Energy, Vol: 3

Journal article

Basso M, Colusso E, Carraro C, Kalha C, Riaz AA, Bombardelli G, Napolitani E, Chen Y, Jasieniak J, Ratcliff LE, Thakur PK, Lee T-L, Regoutz A, Martucci Aet al., 2023, Rapid laser-induced low temperature crystallization of thermochromic VO2 sol-gel thin films, APPLIED SURFACE SCIENCE, Vol: 631, ISSN: 0169-4332

Journal article

Dawson W, Kawashima E, Ratcliff LE, Kamiya M, Genovese L, Nakajima Tet al., 2023, Complexity reduction in density functional theory: Locality in space and energy, JOURNAL OF CHEMICAL PHYSICS, Vol: 158, ISSN: 0021-9606

Journal article

Fernando NK, Stella M, Dawson W, Nakajima T, Genovese L, Regoutz A, Ratcliff LEet al., 2022, Probing disorder in 2CzPN using core and valence states, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 24, Pages: 23329-23339, ISSN: 1463-9076

Journal article

Ratcliff LE, Oshima T, Nippert F, Janzen BM, Kluth E, Goldhahn R, Feneberg M, Mazzolini P, Bierwagen O, Wouters C, Nofal M, Albrecht M, Swallow JEN, Jones LAH, Thakur PK, Lee T, Kalha C, Schlueter C, Veal TD, Varley JB, Wagner MR, Regoutz Aet al., 2022, Tackling Disorder in γ‐Ga<sub>2</sub>O<sub>3</sub>, Advanced Materials, Vol: 34, ISSN: 0935-9648

<jats:title>Abstract</jats:title><jats:p>Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ‐Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure–electronic‐structure relationship. Here, density functional theory is used in combination with a machine‐learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ‐phase. Theoretical results are compared with surface and bulk sensitive soft and hard X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ‐Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.</jats:p>

Journal article

Ratcliff LE, Oshima T, Nippert F, Janzen BM, Kluth E, Goldhahn R, Feneberg M, Mazzolini P, Bierwagen O, Wouters C, Nofal M, Albrecht M, Swallow JEN, Jones LAH, Thakur PK, Lee T-L, Kalha C, Schlueter C, Veal TD, Varley JB, Wagner MR, Regoutz Aet al., 2022, Tackling Disorder in γ-Ga<sub>2</sub>O<sub>3</sub>, ADVANCED MATERIALS, Vol: 34, ISSN: 0935-9648

Journal article

Stella M, Thapa K, Genovese L, Ratcliff LEet al., 2022, Transition-Based Constrained DFT for the Robust and Reliable Treatment of Excitations in Supramolecular Systems, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, Vol: 18, Pages: 3027-3038, ISSN: 1549-9618

Journal article

Dawson W, Degomme A, Stella M, Nakajima T, Ratcliff LE, Genovese Let al., 2022, Density functional theory calculations of large systems: Interplay between fragments, observables, and computational complexity, Wiley Interdisciplinary Reviews: Computational Molecular Science, Vol: 12, Pages: 1-28, ISSN: 1759-0876

In the past decade, developments of computational technology around density functional theory (DFT) calculations have considerably increased the system sizes which can be practically simulated. The advent of robust high performance computing algorithms which scale linearly with system size has unlocked numerous opportunities for researchers. This fact enables computational physicists and chemists to investigate systems of sizes which are comparable to systems routinely considered by experimentalists, leading to collaborations with a wide range of techniques and communities. This has important consequences for the investigation paradigms which should be applied to reduce the intrinsic complexity of quantum mechanical calculations of many thousand atoms. It becomes important to consider portions of the full system in the analysis, which have to be identified, analyzed, and employed as building-blocks from which decomposed physico-chemical observables can be derived. After introducing the state-of-the-art in the large scale DFT community, we will illustrate the emerging research practices in this rapidly expanding field, and the knowledge gaps which need to be bridged to face the stimulating challenge of the simulation of increasingly realistic systems.

Journal article

Ratcliff LE, Genovese L, Park H, Littlewood PB, Lopez-Bezanilla Aet al., 2022, Exploring metastable states in UO<sub>2</sub> using hybrid functionals and dynamical mean field theory, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 34, ISSN: 0953-8984

Journal article

Kalha C, Ratcliff LE, Gutierrez Moreno JJ, Mohr S, Mantsinen M, Fernando NK, Thakur PK, Lee T-L, Tseng H-H, Nunney TS, Kahk JM, Lischner J, Regoutz Aet al., 2022, Lifetime effects and satellites in the photoelectron spectrum of tungsten metal, Physical Review B: Condensed Matter and Materials Physics, Vol: 105, Pages: 1-18, ISSN: 1098-0121

Tungsten (W) is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterization of tungsten and tungsten-based compounds is x-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of tungsten metal, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard x-ray photoelectron spectroscopy (SXPS and HAXPES) with reflected electron energy loss spectroscopy (REELS) and a multitiered ab initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and GW+C), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten-based mixed-metal systems and holds promise for the reassessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.

Journal article

Fernando NK, Cairns AB, Murray CA, Thompson AL, Dickerson JL, Garman EF, Ahmed N, Ratcliff LE, Regoutz Aet al., 2021, Structural and electronic effects of X-ray irradiation on prototypical [M(COD)Cl](2) catalysts, The Journal of Physical Chemistry A: Isolated Molecules, Clusters, Radicals, and Ions; Environmental Chemistry, Geochemistry, and Astrochemistry; Theory, Vol: 125, Pages: 7473-7488, ISSN: 1089-5639

X-ray characterization techniques are invaluable for probing material characteristics and properties, and have been instrumental in discoveries across materials research. However, there is a current lack of understanding of how X-ray-induced effects manifest in small molecular crystals. This is of particular concern as new X-ray sources with ever-increasing brilliance are developed. In this paper, systematic studies of X-ray–matter interactions are reported on two industrially important catalysts, [Ir(COD)Cl]2 and [Rh(COD)Cl]2, exposed to radiation in X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) experiments. From these complementary techniques, changes to structure, chemical environments, and electronic structure are observed as a function of X-ray exposure, allowing comparisons of stability to be made between the two catalysts. Radiation dose is estimated using recent developments to the RADDOSE-3D software for small molecules and applied to powder XRD and XPS experiments. Further insights into the electronic structure of the catalysts and changes occurring as a result of the irradiation are drawn from density functional theory (DFT). The techniques combined here offer much needed insight into the X-ray-induced effects in transition-metal catalysts and, consequently, their intrinsic stabilities. There is enormous potential to extend the application of these methods to other small molecular systems of scientific or industrial relevance.

Journal article

Kalha C, Bichelmaier S, Fernando NK, Berens JV, Thakur PK, Lee T-L, Gutiérrez Moreno JJ, Mohr S, Ratcliff LE, Reisinger M, Zechner J, Nelhiebel M, Regoutz Aet al., 2021, Thermal and oxidation stability of TixW1−x diffusion barriers investigated by soft and hard x-ray photoelectron spectroscopy, Journal of Applied Physics, Vol: 129, Pages: 1-15, ISSN: 0021-8979

The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO

Journal article

Okenyi MTO, Ratcliff LE, Walsh A, 2021, Multi-phonon proton transfer pathway in a molecular organic ferroelectric crystal, Physical Chemistry Chemical Physics, Vol: 23, Pages: 2885-2890, ISSN: 1463-9076

While the majority of ferroelectrics research has been focused on inorganic ceramics, molecular ferroelectrics can also combine large spontaneous polarization with high Curie temperatures. However, the microscopic mechanism of their ferroelectric switching is not fully understood. We explore proton tautomerism in the prototypical case of croconic acid, C5O5H2. In order to determine how efficiently ferroelectricity in croconic acid is described in terms of its Γ-point phonon modes, the minimum energy path between its structural ground states is approximated by projection onto reduced basis sets formed from subsets of these modes. The potential energy curve along the minimum energy path was found to be sensitive to the order of proton transfer, which requires a large subset (≳8) of the modes to be approximated accurately. Our findings suggest rules for the construction of effective Hamiltonians to describe proton transfer ferroelectrics.

Journal article

Regoutz A, Wolinska MS, Fernando NK, Ratcliff LEet al., 2021, A combined density functional theory and x-ray photoelectron spectroscopy study of the aromatic amino acids, Electronic Structure, Vol: 2, Pages: 1-11, ISSN: 2516-1075

Amino acids are essential to all life. However, our understanding of some aspects of their intrinsic structure, molecular chemistry, and electronic structure is still limited. In particular the nature of amino acids in their crystalline form, often essential to biological and medical processes, faces a lack of knowledge both from experimental and theoretical approaches. An important experimental technique that has provided a multitude of crucial insights into the chemistry and electronic structure of materials is x-ray photoelectron spectroscopy. While the interpretation of spectra of simple bulk inorganic materials is often routine, interpreting core level spectra of complex molecular systems is complicated to impossible without the help of theory. We have previously demonstrated the ability of density functional theory to calculate binding energies of simple amino acids, using ΔSCF implemented in a systematic basis set for both gas phase (multiwavelets) and solid state (plane waves) calculations. In this study, we use the same approach to successfully predict and rationalise the experimental core level spectra of phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His) and gain an in-depth understanding of their chemistry and electronic structure within the broader context of more than 20 related molecular systems. The insights gained from this study provide significant information on the nature of the aromatic amino acids and their conjugated side chains.

Journal article

Ratcliff L, Dawson W, Fisicaro G, Caliste D, Mohr S, Degomme A, Videau B, Cristiglio V, Stella M, D'Alessandro M, Goedecker S, Nakajima T, Deutsch T, Genovese Let al., 2020, Flexibilities of wavelets as a computational basis set for large-scale electronic structure calculations, Journal of Chemical Physics, Vol: 152, Pages: 1-28, ISSN: 0021-9606

The BigDFT project was started in 2005 with the aim of testing the advantages of using a Daubechies wavelet basis set for Kohn–Sham (KS) density functional theory (DFT) with pseudopotentials. This project led to the creation of the BigDFT code, which employs a computational approach with optimal features of flexibility, performance, and precision of the results. In particular, the employed formalism has enabled the implementation of an algorithm able to tackle DFT calculations of large systems, up to many thousands of atoms, with a computational effort that scales linearly with the number of atoms. In this work, we recall some of the features that have been made possible by the peculiar properties of Daubechies wavelets. In particular, we focus our attention on the usage of DFT for large-scale systems. We show how the localized description of the KS problem, emerging from the features of the basis set, is helpful in providing a simplified description of large-scale electronic structure calculations. We provide some examples on how such a simplified description can be employed, and we consider, among the case-studies, the SARS-CoV-2 main protease.

Journal article

Dawson W, Mohr S, Ratcliff LE, Nakajima T, Genovese Let al., 2020, Complexity reduction in density functional theory calculations of large systems: system partitioning and fragment embedding., Journal of Chemical Theory and Computation, Vol: 16, Pages: 2952-2964, ISSN: 1549-9618

With the development of low order scaling methods for performing Kohn-Sham density functional theory, it is now possible to perform fully quantum mechanical calculations of systems containing tens of thousands of atoms. However, with an increase in the size of the system treated comes an increase in complexity, making it challenging to analyze such large systems and determine the cause of emergent properties. To address this issue, in this paper, we present a systematic complexity reduction methodology which can break down large systems into their constituent fragments and quantify interfragment interactions. The methodology proposed here requires no a priori information or user interaction, allowing a single workflow to be automatically applied to any system of interest. We apply this approach to a variety of different systems and show how it allows for the derivation of new system descriptors, the design of QM/MM partitioning schemes, and the novel application of graph metrics to molecules and materials.

Journal article

Prentice J, Aarons J, Womack JC, Allen AEA, Andrinopoulos L, Anton L, Bell RA, Bhandari A, Bramley GA, Charlton R, Clements RJ, Cole DJ, Constantinescu G, Corsetti F, Dubois SM-M, Duff KKB, Escartín JM, Greco A, Hill Q, Lee LP, Linscott E, ORegan DD, Phipps MJS, Ratcliff L, Serrano ÁR, Tait EW, Teobaldi G, Vitale V, Yeung N, Zuehlsdorff T, Dziedzic J, Haynes PD, Hine N, Mostofi AA, Payne MC, Skylaris C-Ket al., 2020, The ONETEP linear-scaling density functional theory program, The Journal of Chemical Physics, Vol: 152, Pages: 174111-1-174111-36, ISSN: 0021-9606

We present an overview of the onetep program for linear-scaling density functional theory (DFT) calculations with large basis set (plane-wave) accuracy on parallel computers. The DFT energy is computed from the density matrix, which is constructed from spatially localized orbitals we call Non-orthogonal Generalized Wannier Functions (NGWFs), expressed in terms of periodic sinc (psinc) functions. During the calculation, both the density matrix and the NGWFs are optimized with localization constraints. By taking advantage of localization, onetep is able to perform calculations including thousands of atoms with computational effort, which scales linearly with the number or atoms. The code has a large and diverse range of capabilities, explored in this paper, including different boundary conditions, various exchange–correlation functionals (with and without exact exchange), finite electronic temperature methods for metallic systems, methods for strongly correlated systems, molecular dynamics, vibrational calculations, time-dependent DFT, electronic transport, core loss spectroscopy, implicit solvation, quantum mechanical (QM)/molecular mechanical and QM-in-QM embedding, density of states calculations, distributed multipole analysis, and methods for partitioning charges and interactions between fragments. Calculations with onetep provide unique insights into large and complex systems that require an accurate atomic-level description, ranging from biomolecular to chemical, to materials, and to physical problems, as we show with a small selection of illustrative examples. onetep has always aimed to be at the cutting edge of method and software developments, and it serves as a platform for developing new methods of electronic structure simulation. We therefore conclude by describing some of the challenges and directions for its future developments and applications.

Journal article

Zaccaria M, Dawson W, Cristiglio V, Reverberi M, Ratcliff LE, Nakajima T, Genovese L, Momeni Bet al., 2020, Designing a bioremediator: mechanistic models guide cellular and molecular specialization, CURRENT OPINION IN BIOTECHNOLOGY, Vol: 62, Pages: 98-105, ISSN: 0958-1669

Journal article

Pi JM, Stella M, Fernando NK, Lam AY, Regoutz A, Ratcliff LEet al., 2020, Predicting core level photoelectron spectra of amino acids using density functional theory., Journal of Physical Chemistry Letters, Vol: 11, Pages: 2256-2262, ISSN: 1948-7185

Core level photoelectron spectroscopy is a widely used technique to study amino acids. Interpretation of the individual contributions from functional groups and their local chemical environments to overall spectra requires both high-resolution reference spectra and theoretical insights, for example, from density functional theory calculations. This is a particular challenge for crystalline amino acids due to the lack of experimental data and the limitation of previous calculations to gas phase molecules. Here, a state of the art multiresolution approach is used for high-precision gas phase calculations and to validate core hole pseudopotentials for plane-wave calculations. This powerful combination of complementary numerical techniques provides a framework for accurate ΔSCF calculations for molecules and solids in systematic basis sets. It is used to successfully predict C and O 1s core level spectra of glycine, alanine, and serine and identify chemical state contributions to experimental spectra of crystalline amino acids.

Journal article

Pi J, Stella M, Fernando N, Lam A, Regoutz A, Ratcliff Let al., 2020, Predicting Core Level Photoelectron Spectra of Amino Acids Using Density Functional Theory

<jats:p>Core level photoelectron spectroscopy is a widely used technique to study amino acids. Interpretation of the individual contributions from functional groups and their local chemical environments to overall spectra requires both high-resolution reference spectra and theoretical insights, for example from density functional theory calculations. This is a particular challenge for crystalline amino acids due to the lack of experimental data and the limitation of previous calculations to gas phase molecules. Here, a state of the art multiresolution approach is used for high precision gas phase calculations and to validate core hole pseudopotentials for plane-wave calculations. This powerful combination of complementary numerical techniques provides a framework for accurate ΔSCF calculations for molecules and solids in systematic basis sets. It is used to successfully predict C and O 1<jats:italic>s</jats:italic> core level spectra of glycine, alanine and serine and identify chemical state contributions to experimental spectra of crystalline amino acids.</jats:p>

Journal article

Ratcliff LE, Genovese L, 2020, Enhancing the Flexibility of First Principles Simulations of Materials via Wavelets, THEORY AND SIMULATION IN PHYSICS FOR MATERIALS APPLICATIONS: CUTTING-EDGE TECHNIQUES IN THEORETICAL AND COMPUTATIONAL MATERIALS SCIENCE, Editors: Levchenko, Dappe, Ori, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 57-78, ISBN: 978-3-030-37789-2

Book chapter

Ratcliff LE, Thornton WS, Mayagoitia AV, Romero NAet al., 2019, Combining pseudopotential and all electron density functional theory for the efficient calculation of core spectra using a multiresolution approach, Journal of Physical Chemistry A, Vol: 123, Pages: 4465-4474, ISSN: 1089-5639

Broadly speaking, the calculation of core spectra such as electron energy loss spectra (EELS) at the level of density functional theory (DFT) usually relies on one of two approaches: conceptually more complex but computationally efficient projector augmented wave based approaches or more straightforward but computationally more intensive all electron (AE) based approaches. In this work we present an alternative method, which aims to find a middle ground between the two. Specifically, we have implemented an approach in the multiwavelet madness molecular DFT code that permits a combination of atoms treated at the AE and pseudopotential (PSP) level. Atoms for which one wishes to calculate the core edges are thus treated at an AE level, while the remainder can be treated at the PSP level. This is made possible thanks to the multiresolution approach of madness, which permits accurate and efficient calculations at both the AE and PSP level. Through examples of a small molecule and a carbon nanotube, we demonstrate the potential applications of our approach.

Journal article

Ratcliff LE, Genovese L, 2019, Pseudo-fragment approach for extended systems derived from linear-scaling DFT, Journal of Physics: Condensed Matter, Vol: 31, ISSN: 0953-8984

We present a computational approach which is tailored for reducing the complexity of the description of extended systems at the density functional theory level. We define a recipe for generating a set of localized basis functions which are optimized either for the accurate description of pristine, bulk-like Wannier functions, or for the in situ treatment of deformations induced by defective constituents such as boundaries or impurities. Our method enables one to identify the regions of an extended system which require dedicated optimization of the Kohn–Sham degrees of freedom, and provides the user with a reliable estimation of the errors—if any—induced by the locality of the approach. Such a method facilitates on the one hand an effective reduction of the computational degrees of freedom needed to simulate systems at the nanoscale, while in turn providing a description that can be straightforwardly put in relation to effective models, like tight binding Hamiltonians. We present our methodology with SiC nanotube-like cages as a test bed. Nonetheless, the wavelet-based method employed in this paper makes possible calculation of systems with different dimensionalities, including slabs and fully periodic systems.

Journal article

Ratcliff LE, Conduit GJ, Hine NDM, Haynes PDet al., 2018, Band structure interpolation using optimized local orbitals from linear-scaling density functional theory, Physical Review B, Vol: 98, ISSN: 2469-9950

© 2018 American Physical Society. Several approaches to linear-scaling density functional theory (LS-DFT) that seek to achieve accuracy equivalent to plane-wave methods do so by optimizing in situ a set of local orbitals in terms of which the density matrix can be accurately expressed. These local orbitals, which can also accurately represent the canonical Kohn-Sham orbitals, qualitatively resemble the maximally localized Wannier functions employed in band structure interpolation. As LS-DFT methods are increasingly being used in real-world applications demanding accurate band structures, it is natural to question the extent to which these optimized local orbitals can provide sufficient accuracy. In this paper, we present and compare, in principle and in practice, two methods for obtaining band structures. We apply these to a (10, 0) carbon nanotube as an example. By comparing with the results from a traditional plane-wave pseudopotential calculation, the optimized local orbitals are found to provide an excellent description of the occupied bands and some low-lying unoccupied bands, with consistent agreement across the Brillouin zone. However free-electron-like states derived from weakly bound states independent of the σ and π orbitals can only be found if additional local orbitals are included.

Journal article

Shin H, Benali A, Luo Y, Crabb E, Lopez-Bezanilla A, Ratcliff LE, Jokisaari AM, Heinonen Oet al., 2018, Zirconia and hafnia polymorphs: Ground-state structural properties from diffusion Monte Carlo, Physical Review Materials, Vol: 2, ISSN: 2475-9953

Zirconia (zirconium dioxide) and hafnia (hafnium dioxide) are binary oxides used in a range of applications. Because zirconium and hafnium are chemically equivalent, they have three similar polymorphs, and it is important to understand the properties and energetics of these polymorphs. However, while density functional theory calculations can get the correct energetic ordering, the energy differences between polymorphs depend very much on the specific density functional theory approach, as do other quantities such as lattice constants and bulk modulus. We have used highly accurate quantum Monte Carlo simulations to model the three zirconia and hafnia polymorphs. We compare our results for structural parameters, bulk modulus, and cohesive energy with results obtained from density functional theory calculations. We also discuss comparisons of our results with existing experimental data, in particular for structural parameters where extrapolation to zero temperature can be attempted. We hope our results of structural parameters as well as for cohesive energy and bulk modulus can serve as benchmarks for density-functional theory based calculations and as a guidance for future experiments.

Journal article

Ratcliff LE, Degomme A, Flores-Livas JA, Goedecker S, Genovese Let al., 2018, Affordable and accurate large-scale hybrid-functional calculations on GPU-accelerated supercomputers, Journal of Physics: Condensed Matter, Vol: 30, Pages: 1-11, ISSN: 0953-8984

Performing high accuracy hybrid functional calculations for condensed matter systems containing a large number of atoms is at present computationally very demanding or even out of reach if high quality basis sets are used. We present a highly optimized multiple graphics processing unit implementation of the exact exchange operator which allows one to perform fast hybrid functional density-functional theory (DFT) calculations with systematic basis sets without additional approximations for up to a thousand atoms. With this method hybrid DFT calculations of high quality become accessible on state-of-the-art supercomputers within a time-to-solution that is of the same order of magnitude as traditional semilocal-GGA functionals. The method is implemented in a portable open-source library.

Journal article

Mohr S, Masella M, Ratcliff LE, Genovese Let al., 2017, Complexity reduction in large quantum systems: fragment identification and population analysis via a local optimized minimal basis., Journal of Chemical Theory and Computation, Vol: 13, Pages: 4079-4088, ISSN: 1549-9618

We present, within Kohn-Sham density functional theory calculations, a quantitative method to identify and assess the partitioning of a large quantum-mechanical system into fragments. We then show how within this framework simple generalizations of other well-known population analyses can be used to extract, from first-principles, reliable electrostatic multipoles for the identified fragments. Our approach reduces arbitrariness in the fragmentation procedure and enables the possibility to assess quantitatively whether the corresponding fragment multipoles can be interpreted as observable quantities associated with a system moiety. By applying our formalism within the code BigDFT, we show that the use of a minimal set of in situ-optimized basis functions allows at the same time a proper fragment definition and an accurate description of the electronic structure.

Journal article

Pelzer KM, Vázquez-Mayagoitia Á, Ratcliff LE, Tretiak S, Bair RA, Gray SK, Van Voorhis T, Larsen RE, Darling SBet al., 2017, Molecular dynamics and charge transport in organic semiconductors: a classical approach to modeling electron transfer., Chem Sci, Vol: 8, Pages: 2597-2609, ISSN: 2041-6520

Organic photovoltaics (OPVs) are a promising carbon-neutral energy conversion technology, with recent improvements pushing power conversion efficiencies over 10%. A major factor limiting OPV performance is inefficiency of charge transport in organic semiconducting materials (OSCs). Due to strong coupling with lattice degrees of freedom, the charges form polarons, localized quasi-particles comprised of charges dressed with phonons. These polarons can be conceptualized as pseudo-atoms with a greater effective mass than a bare charge. We propose that due to this increased mass, polarons can be modeled with Langevin molecular dynamics (LMD), a classical approach with a computational cost much lower than most quantum mechanical methods. Here we present LMD simulations of charge transfer between a pair of fullerene molecules, which commonly serve as electron acceptors in OSCs. We find transfer rates consistent with experimental measurements of charge mobility, suggesting that this method may provide quantitative predictions of efficiency when used to simulate materials on the device scale. Our approach also offers information that is not captured in the overall transfer rate or mobility: in the simulation data, we observe exactly when and why intermolecular transfer events occur. In addition, we demonstrate that these simulations can shed light on the properties of polarons in OSCs. Much remains to be learned about these quasi-particles, and there are no widely accepted methods for calculating properties such as effective mass and friction. Our model offers a promising approach to exploring mass and friction as well as providing insight into the details of polaron transport in OSCs.

Journal article

Ratcliff LE, Mohr S, Huhs G, Deutsch T, Masella M, Genovese Let al., 2017, Challenges in large scale quantum mechanical calculations, Wiley Interdisciplinary Reviews: Computational Molecular Science, Vol: 7, ISSN: 1759-0876

© 2016 John Wiley & Sons, Ltd During the past decades, quantum mechanical methods have undergone an amazing transition from pioneering investigations of experts into a wide range of practical applications, made by a vast community of researchers. First principles calculations of systems containing up to a few hundred atoms have become a standard in many branches of science. The sizes of the systems which can be simulated have increased even further during recent years, and quantum-mechanical calculations of systems up to many thousands of atoms are nowadays possible. This opens up new appealing possibilities, in particular for interdisciplinary work, bridging together communities of different needs and sensibilities. In this review we will present the current status of this topic, and will also give an outlook on the vast multitude of applications, challenges, and opportunities stimulated by electronic structure calculations, making this field an important working tool and bringing together researchers of many different domains. WIREs Comput Mol Sci 2017, 7:e1290. doi: 10.1002/wcms.1290. For further resources related to this article, please visit the WIREs website.

Journal article

Elliott JD, Poli E, Scivetti I, Ratcliff LE, Andrinopoulos L, Dziedzic J, Hine NDM, Mostofi AA, Skylaris C-K, Haynes PD, Teobaldi Get al., 2016, Chemically selective alternatives to photoferroelectrics for polarization-enhanced photocatalysis: the untapped potential of hybrid inorganic nanotubes, Advanced Science, Vol: 4, ISSN: 2198-3844

Linear-scaling density functional theory simulation of methylated imogolite nanotubes (NTs) elucidates the interplay between wall-polarization, bands separation, charge-transfer excitation, and tunable electrostatics inside and outside the NT-cavity. The results suggest that integration of polarization-enhanced selective photocatalysis and chemical separation into one overall dipole-free material should be possible. Strategies are proposed to increase the NT polarization for maximally enhanced electron–hole separation.

Journal article

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