High performance computing is a powerful tool to accelerate the Kohn–Sham density functional theory calculations on modern heterogeneous supercomputers. Here, we describe a massively parallel implementation of large-scale linear-response time-dependent density functional theory (LR-TDDFT) to calculate the excitation energies and wave functions of solids with plane-wave basis set. We adopt a two-level parallelization strategy that combines the message passing interface with open multi-processing parallel programming to deal with the matrix operations and data communications of constructing and diagonalizing the LR-TDDFT Hamiltonian matrix. Numerical results illustrate that the LR-TDDFT calculations can scale up to 24 576 processing cores on modern heterogeneous supercomputers to study the excited state properties of bulky silicon systems containing thousands of atoms (4,096 atoms). We demonstrate that the LR-TDDFT calculations can be used to investigate the photoinduced charge separation of water molecule adsorption on rutile TiO2(110) surface from an excitonic perspective.
Emerging Leaders 2020
Scope
EST is proud to represent the electronic structure community, and as such, we are will be publishing our first Emerging Leaders focus issue, bringing together the best early-career researchers from all areas of electronic structure research.
An emerging leader is defined as a top researcher who completed their PhD in 2010 or later (10 years excluding career breaks). A limited number of early career researchers have been nominated by the journal's Editorial Board as the most talented and exciting researchers in their generation. Our congratulations to all those nominated!
This special issue will cover a vast range of topics covered within the scope of EST, and will hopefully reflect the breadth of modern condensed matter physics. The submission window will be open until 30 September 2020.
Wei Hu
Dr. Wei Hu focuses on the development, implementation and application of large-scale density functional theory (DFT) methods, such as new algorithms for accelerating the DFT, HF, TDDFT and GWBSE calculations (ISDF, ACE and PCDIIS), and high-performance computing DFT software development (HONPAS, KSSOLV, DGDFT and PWDFT). He also use these methods for materials simulations and design, such as surface catalysis, photocatalysis, gas sensors, solar cells, FETs, Schottky diodes and PN junctions. His research goal is to push the envelope of ultra-large-scale materials simulations containing tens of thousands of atoms on modern heterogeneous supercomputers.
Laura Ratcliff
Dr. Laura Ratcliff is an EPSRC Early Career Research Fellow in the Department of Materials at Imperial College London. She has an MPhys in Theoretical Physics (with a year in Europe) from the University of York (2008) and a PhD from the Department of Materials at Imperial College London (2012). She held postdoctoral positions at CEA Grenoble, France, and Argonne National Laboratory, US, before returning to Imperial College (2017). She develops and applies new methods based on density functional theory (DFT), with interests in large systems and theoretical spectroscopy, and has contributed to three DFT codes — BigDFT, ONETEP and MADNESS.
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.
Nicholas Bristowe
Nicholas Bristowe's group investigates structure-property relations in perovskites and related functional materials. He recently joined Durham University, his alma mater, as an Assistant Professor in Theoretical Condensed Matter Physics. His interest in oxide perovskites began during his PhD on the LaAlO3-SrTiO3 interface at the University of Cambridge. He was later awarded a research fellowship from the Royal Commission for the Exhibition of 1851, and subsequently an Imperial College Research Fellowship, to primarily study multiferroic materials. In 2017 he obtained a Lectureship at the University of Kent, before moving to Durham in 2020, and remains an honorary lecturer at Imperial.
ABO3 perovskites host a huge range of symmetry lowering structural distortions, each of which can tune, or even switch on or off, different functional properties due to the strong coupling between the lattice, spin and charge degrees of freedom in these materials. The sheer number of different meta-stable structures present in perovskites creates a challenge for materials design via theory and simulation. Here, we tackle this issue using a first principles structure searching method on a prototypical half-metallic perovskite, La0.7Sr0.3MnO3, to predict how epitaxial strain can engineer structural and magnetic properties. We reveal a rich structural phase diagram through strain engineering in which the octahedral tilt pattern, and hence the crystal symmetry, is altered from the bulk. We show how the low-symmetry of the various phases in turn induces new structural modes, an increase in the magnetic anisotropy energy, and weak antiferromagnetic spin-canting.
Zhendong Li
Dr. Zhendong Li obtained his B.S. in Chemistry and Mathematics at the Peking University, China, in 2009, and received Ph.D. at the Peking University in Theoretical Chemistry in 2014. Then, he worked as a postdoc in Princeton University and Caltech. In 2019, he joined Beijing Normal University as a professor in College of Chemistry. His primary interests lie in the development of accurate and efficient electronic structure methods for strongly correlated transition metal complexes using tensor networks states and quantum computations.
Polynuclear transition metal complexes such as the P-cluster and the FeMo-cofactor of nitrogenase with eight transition metal centers represent a great challenge for current electronic structure methods. In this work, we initiated the use of comb tensor network states (CTNS), whose underlying topology has a one-dimensional backbone and several one-dimensional branches, as a many-body wavefunction ansatz to tackle these challenging systems. As an important first step, we explored the expressive power of CTNS with different underlying topologies. To this end, we presented an algorithm to express a configuration interaction (CI) wavefunction into CTNS based on the Schmidt decomposition. The algorithm was illustrated for representing approximate CI wavefunctions obtained from selected CI calculations for the P-cluster and the FeMo-cofactor into CTNS with three chemically meaningful comb structures, which successively group orbitals belonging to the same atom into branches. The conventional matrix product states (MPS) representation was obtained as a special case. We also discussed the insights gained from such decompositions, which shed some light on the future developments of efficient numerical tools for polynuclear transition metal complexes.
Renana Gershoni-Poranne
Renana Gershoni-Poranne received a BSc in Molecular Biochemistry (summa cum laude) and an MSc in Organic Chemistry (summa cum laude) from the Technion — Israel Institute of Technology. She then completed her PhD in Physical Organic Chemistry under the supervision of Prof. Amnon Stanger in the same institute. In 2015 she joined the group of Prof. Peter Chen at the Laboratory for Organic Chemistry at the ETH Zurich for her postdoctoral research. Since 2017 she has been a Group Leader/Senior Scientist within Prof. Chen's group. Her group focuses on understanding the reactivity and electronic properties of polycyclic aromatic systems, including development of intuitive tools for characterization of such properties. In 2019 Dr. Gershoni-Poranne was awarded a Branco Weiss Fellowship to extend this work into new approaches for molecular design of organic electronics.
Polycyclic aromatic systems are prevalent in chemistry and materials science because their thermodynamic stability, planarity, and tunable electronic properties make them uniquely suited for various uses. These properties are closely linked to the aromaticity of the systems. Therefore, characterizing the aromatic behavior is useful for designing new functional compounds and understanding their reactivity. NICS-XY-scans are a popular and simple tool for investigating the aromatic trends in polycyclic systems. Herein we present Predi-XY: an automated system for generating NICS-XY-scans for polycyclic aromatic systems using an additivity scheme. The program provides the predicted scans at a fraction of the computational cost of a full quantum mechanical calculation and enables rapid comparison of various polycyclic aromatic systems.
Joseph Barker
Joseph Barker is a Royal Society University Research Fellowship at the University of Leeds. Prior to this he was an assistant professor in Tohoku University, Japan for five years, working on the topics of insulator spintronics and spincaloritronics. He completed his PhD on the topic of ultrafast magnetisation dynamics at the University of York. Joseph specialises in atomistic spin modelling. His current research is in making quantitative, material specific calculations of thermal and spin transport effects in magnetic materials. He often collaborates with experts in the electronic structure and neutron scattering communities to parametrise magnetic Hamiltonians for modelling.
Yttrium iron garnet is a complex ferrimagnetic insulator with 20 magnon modes which is used extensively in fundamental experimental studies of magnetisation dynamics. As a transition metal oxide with moderate gap (2.8 eV), yttrium iron garnet requires a careful treatment of electronic correlation. We have applied quasiparticle self-consistent GW to provide a fully ab initio description of the electronic structure and resulting magnetic properties, including the parameterisation of a Heisenberg model for magnetic exchange interactions. Subsequent spin dynamical modelling with quantum statistics extends our description to the magnon spectrum and thermodynamic properties such as the Curie temperature, finding favourable agreement with experimental measurements. This work provides a snapshot of the state-of-the art in modelling of complex magnetic insulators.
Chen Huang
Chen Huang received his bachelor's degree in physics in 2003 from Tsinghua University and then obtained his Ph.D. in physics in 2011 from Princeton University under the advisement of Professor Emily Carter. He then spent three years working as a postdoctoral research associate under the advisement of Dr. Arthur Voter and Dr. Danny Perez at Los Alamos National Laboratory. In 2014, he joined the Department of Scientific Computing at the Florida State University as an assistant professor. He was promoted to associate professor in 2020. His research focuses on developing quantum mechanical embedding methods for obtaining sufficiently accurate electronic structures in large strongly-correlated materials.
A kernel polynomial method is developed to calculate the random phase approximation (RPA) correlation energy. In the method, the RPA correlation energy is formulated in terms of the matrix that is the product of the Coulomb potential and the density linear response functions. The integration over the matrix's eigenvalues is calculated by expanding the density of states of the matrix in terms of the Chebyshev polynomials. The coefficients in the expansion are obtained through stochastic sampling. Since it is often the energy difference between two systems that is of much interest in practice, another focus of this work is to develop a correlated sampling scheme to accelerate the convergence of the stochastic calculations of the RPA correlation energy difference between two similar systems. The scheme is termed the atom-based correlated sampling (ACS). The performance of ACS is examined by calculating the isomerization energy of acetone to 2-propenol and the energy of the water–gas shift reaction. Using ACS, the convergences of these two examples are accelerated by 3.6 and 4.5 times, respectively. The methods developed in this work are expected to be useful for calculating RPA-level reaction energies for the reactions that take place in local regions, such as calculating the adsorption energies of molecules on transition metal surfaces for modeling surface catalysis.
Jan Marcus Dahlström
I defended my PhD thesis in 2011 with supervisor Prof. Anne L'Huillier from the Atomic Physics Division at Lund University (LTH). During my PhD, I performed both experimental and theoretical work on non-linear Atomic Molecular and Optical (AMO) processes, such as high-order harmonic generation. After my PhD, I have made a solid contribution to the field of attosecond science — the science of capturing electronic motion — by developing a theory for atomic delays in photoionization (see attached Selected Article, X).
After my PhD, I have worked as a Post-doc at Stockholm University (SU) with Prof. Eva Lindroth. The main objective of this project was to perform atomic delay calculations for many-electron correlated atoms subject to attosecond pulse trains (see VIII and IX). I have also worked as a Guest Researcher at the Center for Free-Electron Laser Science (CFEL) in Hamburg, where I proposed new types of pump-probe schemes that use a combination of attosecond pulses and Free-Electron Laser (FEL) beams (see VII). Further, I have been a short-term visitor at Harvard University (see II and V).
In early 2017, I started an Associate Lectureship (BUL) at the Mathematical Physics Division at Lund University, where I was appointed Reader in 2018. My ambition was now to expand my research activities with a Swedish Foundations' Starting Grant later in 2018. This has allowed me to start my own research group. In late 2019, I was appointed Wallenberg Academy Fellow (WAF). Currently, I am pushing the frontier of attosecond metrology by applying and developing advanced numerical techniques for time-dependent atomic physics (see I and III).
A relativistic transient absorption theory is derived, implemented and validated within the dipole approximation based on the time-dependent Dirac equation. In the non-relativistic limit, it is found that the absorption agrees with the well established non-relativistic theory based on the time-dependent Schrödringer equation. Time-dependent simulations have been performed using the Dirac equation and the Schrödinger equation for the hydrogen atom in two different attosecond transient absorption scenarios. These simulations validate the present relativistic theory. The presented work can be seen as a first step in the development of a more general relativistic attosecond transient absorption spectroscopy method for studying heavy atoms, but it also suggests the possibility of studying relativistic effects, such as Zitterbewegung, in the time domain.
Giorgia Fugallo
Giorgia Fugallo is a permanent CNRS researcher at the Thermal and Energy Laboratory of Nantes Polytechnique.
She received her PhD in Physics at King's College London in 2012 and then she was Postdoctoral Researcher at Sorbonne University-Paris VI and EPFL. Prior to join CNRS in 2017, she got back to France in 2014 with a EDF & Ecole Polytechnique “Renewable Energies” Fellowship Grant. Her main research interests focus on the theoretical study, via the state-of-the-art of numerical methods, of material properties for energy applications, ranging from conductive and radiative thermal properties for thermal management applications, to electronic excitations for photovoltaic applications.
Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this work, we focus on the exciton properties of molybdenum sisulfide (MoS2) by solving the ab initio many-body Bethe–Salpeter equation, as a function of momentum, to obtain the excitation spectra of both monolayer and bulk MoS2. We analyse the spectrum and the exciton dispersion on the basis of a model excitonic Hamiltonian capable of providing an efficient description of the excitations in the bulk crystal, starting from the knowledge of the excitons of a single layer. In this way, we obtain a general characterization of both bright and darks excitons in terms of the interplay between the electronic band dispersion (i.e. interlayer hopping) and the electron–hole exchange interaction. We identify for both the 2D and the 3D limiting cases the character of the lowest-energy excitons in MoS2, we explain the effects and relative weights of both band dispersion and electron–hole exchange interaction and finally we interpret the differences observed when changing the dimensionality of the system.
Papers
Electronic structures with the type-II band alignment usually exist only in heterostructures. Using the generalized Bloch theorem, we reveal that an effective type-II band alignment can be induced in the single crystalline TiO2 nanowires (NWs) by an axial twisting deformation. With this, we further reveal distinct responses of the the valence band (VB) states and the conduction band (CB) states in the radial dimension of the nanowire. Specifically, in the twisted NW, the VB states migrate from the NW core toward the NW shell while the CB states migrate from the NW shell toward the NW core. Our results pave a new way to realize the type-II band alignment in single crystalline NWs, which is critical for actual applications such as NW-based photovoltaic effect and photocatalytic effect. Further, the significant variation of the quantum states also illustrates the possibility to tune the electronic properties of TiO2 NWs by strain engineering.