Quantum Crystallography in the Last Decade: Developments and Outlooks

In this review article, we report on the recent progresses in the field of quantum crystallography that has witnessed a massive increase of production coupled with a broadening of the scope in the last decade. It is shown that the early thoughts about extracting quantum mechanical information from crystallographic experiments are becoming reality, although a century after prediction. While in the past the focus was mainly on electron density and related quantities, the attention is now shifting toward determination of wavefunction from experiments, which enables an exhaustive determination of the quantum mechanical functions and properties of a system. Nonetheless, methods based on electron density modelling have evolved and are nowadays able to reconstruct tiny polarizations of core electrons, coupling charge and spin models, or determining the quantum behaviour at extreme conditions. Far from being routine, these experimental and computational results should be regarded with special attention by scientists for the wealth of information on a system that they actually contain

Ab initio Quantum Chemistry Methods for Modeling Molecular Excited States Beyond Configuration Interaction Singles

Electron transfer and energy transfer play a central role in photo-induced excited state chemical dynamics and are critical for understanding the fundamental processes in photosynthesis. Understanding electron and energy transfer at the molecular level is essential, since they must compete with deactivation processes back to the molecular ground state-- and deactivation releases any captured energies as wasted heat. Modeling electronic relaxation process is very challenging, however, for 2 reasons: i) Obtaining accurate potential energy surfaces (PESs) by solving the electronic Hamiltonian (only) is nontrivial, since all electrons are coupled together, which is essentially a many-body problem. It is even more difficult in the context of photochemistry, where the relevant molecules are typically big; ii) The Born-Oppenheimer Approximation of separating electronic and nuclear motion may be invalid, and thus one has to model nonadiabatic dynamics. This thesis is focused on the first problem above, i.e. solving the electronic Hamiltonian, where there is currently a lack of effective ab initio quantum chemistry methods, especially in the presence of charge transfer (CT) states. Historically Configuration Interaction Singles (CIS) has been the standard method for modeling electronic excited states with qualitatively correct wavefunctions, but CIS is highly biased against charge transfer states-- which are very important for modeling photo-induced relaxation. Nevertheless, in this thesis, CIS proves to be a good starting point for improved ab initio quantum chemistry methods, that build in the correct molecular orbital optimization. These algorithms are labeled as: i) Orbital Optimized Configuration Interaction Singles (OO-CIS), ii) Variational Orbital Adapted Configuration Interaction Singles (VOA-CIS), and iii) Fully Variational Orbital Adapted Configuration Interaction Singles (FVOA-CIS). Each of the three algorithms above represents an improvement upon its predecessor. i) OOCIS is able to recover perturbative corrections for CT states; ii) its variational extension VOA-CIS proves to be very effective for constructing globally smooth adiabatic PESs even with CT states; and iii) because it is fully variational, FVOA-CIS PESs are so smooth that it should allow analytic gradients. We believe these approaches will be widely used for future accurate electronic structure calculations

Development and application of ab initio electron dynamics on traditional and quantum compute architectures

Electron dynamics processes are of utmost importance in chemistry. For example, light-induced processes are used in the field of photocatalysis to generate a wide variety of products by charge transfer, bond breaking, or electron solvation. Also in the field of materials science, more and more such processes are known and utilized, for example, to design more efficient solar cells. Even the formation of bonds in molecules is an electron dynamics process. Through experimental progress, it is now even possible to trigger specific processes and chemical reactions with special laser pulses. To study all these processes, computer-aided simulations are an indispensable tool. Depending on the size of the molecules considered and the desired accuracy, however, the underlying quantum-mechanical properties result in numerical formulas whose computation far exceeds the capabilities of even modern supercomputers. In this thesis, three projects are presented to demonstrate modern use cases of electron dynamics and show how recent developments in computer technology and software design can be used to develop more efficient and user-friendly programs. In the first project, the inter-Coulombic decay (ICD), an ultrafast energy transfer process, between two isolated chemical structures is investigated. After the excitation of one structure, the energy is transferred to the other, which is ionized as a result. The process has already been shown experimentally in atoms and molecules and is studied here for quantum dots, focusing on systems with more quantum dots and higher dimensions for the continuum than in previous studies. These elaborate studies are made possible by implementing computationally intensive program parts of the Heidelberg MCTDH program used on graphics processing units (GPUs). The performed studies show how the ICD process behaves with multiple partners as well as which competing decay processes occur and thus provide relevant information for the development of technologies based on quantum dots such as quantum dot qubits for use in quantum computers. Electron dynamics processes are not only relevant in the development of new quantum computers, but conversely, quantum computers can also provide the ability to perform electron dynamics with significantly more interacting electrons and a smaller error than it would ever be possible with traditional computers. In another project, therefore, a quantum algorithm was developed that could enable such simulations and their analysis in the future. The quantum algorithm was implemented in the dynamics program Jellyfish, which was also developed in the context of this dissertation. The program is based on a graphical user interface oriented on dataflow programming, which simultaneously leads to a modular structure. The resulting modules can be combined flexibly, which allows Jellyfish to be used for a wide variety of applications. In addition to dynamic algorithms, novel analysis methods were developed and demonstrated on laser-driven electronic excitations in molecules such as hydrogen, lithium cyanide, or guanine. Thus, the generation of electronic wave packets as well as transitions between electronic states were studied in an explicitly time-dependent manner and the formation of the exciton in such processes was described qualitatively by means of densities as well as quantitatively by so-called exciton descriptors such as exciton size or hole and particle position. Thus, in summary, this dissertation presents both new insights into electron dynamic processes and new possibilities for more efficient simulation of these processes using GPU implementations and quantum algorithms. The developed dynamics program Jellyfish offers the potential to be used in many further studies in this area and to be extended to allow for example simulations with a continuum like in the ICD calculations in the future

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post–Hartree–Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension

Computational study of electron-transfers and singlet oxygen in aprotic metal-O2 batteries

Aprotic metal-oxygen batteries (MOBs), based on the electroreduction of molecular oxygen at a porous cathode, have attracted a vast interest in research, owing to their potential upgrade in terms of energy density and costs over present lithium-ion batteries. Despite their highly promising features, aprotic MOBs based on alkali and alkaline-earth metals still suffer severe limitations in their practical applicability. One of the main unresolved issues, especially with Li-O2 batteries, is represented by the high degree of parasitic reactivity. Singlet oxygen (1O2) is today held responsible for a major contribution to such reactivity, and the disproportionation of the superoxide anion is considered as one of the most likely source of 1O2 in the cell environment. Experimental evidences for electrolyte degradation and evolution of 1O2 have been reported, but the fundamental chemical mechanisms underlying these phenomena are still poorly understood. A valid strategy for contrasting the arise of side-reactions and materials degradation is to use redox mediators (RMs), which allow to recharge the battery with greatly reduced overpotentials. Understanding the con- nection of RM-assisted charging with the production 1O2 is likely to play a key role in the design of fully reversible and efficient practical MOBs in the future. In this thesis, quantum chemical computational methods were used to investigate reactive processes of electron-transfer involving reduced oxygen species in aprotic MOBs. The possibility of reactive pathways leading to the release of 1O2 was addressed in particular. The aim of the thesis was to apply theoretical methods to the modeling of reactive systems, in order to unravel part of the mechanisms which underpin the parasitic chemistry of MOBs. Despite their apparent simplicity, the reaction governing the chemistry of the cells involve a complex interplay of radical species and electronic excited states. For this reason, our approach was to use mainly ab-initio correlated multiconfigurational methods for a high-level description of potential energy surfaces and reaction energies. Owing to the computational costs of the methods, such an approach necessarily entails the resort to simplified models, including the exclusive use of implicit solvent and the neglect of solid phases and interfacial effects

Theoretical study of adsorption, excitation and resonant charge transfer of organic molecules on metal surfaces

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 05-12-2019Esta tesis tiene embargado el acceso al texto completo hasta el 05-06-202

Quantum Chemical Investigation of the Interaction of Hydrogen with Solid Surfaces

In dieser Arbeit werden die Wechselwirkungen von Wasserstoff mit festen Materialien und Oberflächen untersucht. Zunächst wird der Kontext unserer Untersuchung durch eine kurze Einordnung in die Geschichte der Naturwissenschaften im Allgemeinen, und der Oberflächenforschung im Speziellen, hergestellt. Anschließend wird der quanten- mechanische Apparat, welcher nötig ist um die betrachteten Systeme zu beschreiben, eingeführt um dann detailliert die Potentialhyperfläche der Entstehung von Wasser durch Adsoprtion von Wasserstoff auf einer teilweise oxidierten Ruthenium(0001) Metalloberfläche zu studieren. Zudem wird das gleiche System betrachtet, wenn die Metalloberfläche zusätzlich von einer biatomaren, kristallinen Lage Siliziumdioxid (SiO2 ) bedeckt ist, wodurch eine räumliche Beengung eintritt. Wir verwenden unsere Ergebnisse zusammen mit experimentellen Beobachtungen und mathematischen Metho- den um ein vollständig theoretisches Modell zu entwerfen und das System grundlegend verstehen zu können. In einem weiteren Schritt werden die chemischen Änderungen der Siliziumdioxid Doppellage untersucht, wenn das System Wasserstoffplasma ausgesetzt wird. Es werden diverse mögliche Defektstrukturen diskutiert und mithilfe experi- menteller Befunde die wahrscheinlichste Struktur isoliert. Im letzten Kapitel werden die typischen Näherungen untersucht, welche notwendig sind um quantenmechanische Methoden mit Hilfe von Computern durchführbar zu machen. Wir verwenden den sogenannten embedded-fragment Ansatz um die Diffusionsbarriere von Wasserstoff auf Aluminiumoxid mit chemischer Genauigkeit zu berechnen. Unsere Ergebnisse auf dem coupled-cluster with singles, doubles and perturbative triples (CCSD(T))- Niveau können sowohl als Referenz für experimentelle Untersuchungen, als auch für andere quantenmechanische Methoden wie z.B. die Dichtefunktionaltheorie, angesehen werden.The present thesis aims at investigating the interactions of hydrogen with solid surfaces and materials. We first offer a brief historical context for surface science, as well as quantum mechanics and science is general, before deriving the mathematical appa- ratus necessary to investigate our systems of interest. We then move on to explore the potential energy surface of the water-formation-reaction on a partially oxidized ruthenium(0001) surface when confined under a two-atom thick sheet of silica (SiO2 ). We further employ our findings in conjunction with experimental observations and mathematical modeling to set up a fully theoretical model of the system in order to explain its behavior. In the second chapter we investigate the chemical alteration of the ultra-thin silica bilayer by means of exposing it to hydrogen plasma. We elucidate possible defects formed during the process and pin-point the most likely structure found. In the last chapter, we investigate the possible error sources that are inherent in quantum mechanical modeling and employ the so called embedded fragment approach to lift the approximations up to the coupled cluster singles and doubles with perturba- tive triples (CCSD(T)) level of theory. We then apply this methodology to the diffusion of hydrogen on aluminum oxide to obtain a diffusion barrier of chemical accuracy that may both be used to benchmark other approaches such as density functional theory, as well as experimental findings

New architectures and designs for organic photovoltaics

The field of organic photovoltaics has seen many significant findings over the last two decades. It is now a very active area of research generating thousands of publications, resulting in advancements in a multi-disciplinary setting. The research described in the body of this thesis aims to investigate the use of new materials and architectures in the fabrication of organic photovoltaics. An approach to incorporate carbon nanotube-Buckminster fullerene hybrid materials into the blended active layer of organic photovoltaic devices is introduced and the effects on devices are elucidated. It is found that the use of cut (barrel) single walled carbon nanotubes was the least detrimental to device performance. The use of alternative methods to fabricate or replace commonly used materials and inter-layers (PCBM, ITO, PEDOT:PSS) in the device structure are presented. The use of thin metal films such as silver and gold is shown to be viable and interesting alternatives to ITO. Layer-by-layer assembly of PEDOT:PSS and electrochemically deposited alternatives are found to have similar performance to standard devices. The fabrication and characterization of a new vertically orientated organic photovoltaic device architecture, the stack device, is presented with a proposed optical model to describe the experimental findings. In particular, the optical mechanism responsible for the operation of the stack device is determined to be frustrated total internal reflection. The new architecture is applied to the fabrication of devices using the standard P3HT:PCBM active layer resulting in an increase in performance

Simulations of charge transport in organic compounds

To aid the design of organic semiconductors, we study the charge transport properties of organic liquid crystals, i.e. hexabenzocoronene and carbazole macrocycle, and single crystals, i.e. rubrene, indolocarbazole and benzothiophene derivatives (BTBT, BBBT). The aim is to find structure-property relationships linking the chemical structure as well as the morphology with the bulk charge carrier mobility of the compounds. To this end, molecular dynamics (MD) simulations are performed yielding realistic equilibrated morphologies. Partial charges and molecular orbitals are calculated based on single molecules in vacuum using quantum chemical methods. The molecular orbitals are then mapped onto the molecular positions and orientations, which allows calculation of the transfer integrals between nearest neighbors using the molecular orbital overlap method. Thus we obtain realistic transfer integral distributions and their autocorrelations. In case of organic crystals the differences between two descriptions of charge transport, namely semi-classical dynamics (SCD) in the small polaron limit and kinetic Monte Carlo (KMC) based on Marcus rates, are studied. The liquid crystals are investigated solely in the hopping limit. To simulate the charge dynamics using KMC, the centers of mass of the molecules are mapped onto lattice sites and the transfer integrals are used to compute the hopping rates. In the small polaron limit, where the electronic wave function is spread over a limited number of neighboring molecules, the Schroedinger equation is solved numerically using a semi-classical approach. The results are compared for the different compounds and methods and, where available, with experimental data. The carbazole macrocycles form columnar structures arranged on a hexagonal lattice with side chains facing inwards, so columns can closely approach each other allowing inter-columnar and thus three-dimensional transport. When taking only intra-columnar transport into account, the mobility is orders of magnitude lower than in the three-dimensional case. BTBT is a promising material for solution-processed organic field-effect transistors. We are able to show that, on the time-scales of charge transport, static disorder due to slow side chain motions is the main factor determining the mobility. The resulting broad transfer integral distributions modify the connectivity of the system but sufficiently many fast percolation paths remain for the charges. Rubrene, indolocarbazole and BBBT are examples of crystals without significant static disorder. The high mobility of rubrene is explained by two main features: first, the shifted cofacial alignment of its molecules, and second, the high center of mass vibrational frequency. In comparsion to SCD, only KMC based on Marcus rates is capable of describing neighbors with low coupling and of taking static disorder into account three-dimensionally. Thus it is the method of choice for crystalline systems dominated by static disorder. However, it is inappropriate for the case of strong coupling and underestimates the mobility of well-ordered crystals. SCD, despite its one-dimensionality, is valuable for crystals with strong coupling and little disorder. It also allows correct treatment of dynamical effects, such as intermolecular vibrations of the molecules. Rate equations are incapable of this, because simulations are performed on static snapshots. We have thus shown strengths and weaknesses of two state of the art models used to study charge transport in organic compounds, partially developed a program to compute and visualize transfer integral distributions and other charge transport properties, and found structure-mobility relations for several promising organic semiconductors.Um die Herstellung organischer Halbleiter zu erleichtern, untersuchen wir den Ladungstransport in organischen Fluessigkristallen, wie Hexabenzocoronen und Carbazolringen, und Einkristallen, wie Rubren, Indolocarbazol und Benzothiophenderivaten. Zielsetzung ist es, Zusammenhaenge zwischen der Struktur und der Ladungstraegermobilitaet zu finden. Zu diesem Zweck werden Molekulardynamiksimulationen (MD) durchgefuehrt, welche realistische, equilibrierte Morphologien liefern. Partialladungen und Elektronenorbitale werden an Einzelmolekuelen im Vakuum mit quantenchemischen Methoden berechnet. Die Orbitale werden dann auf die Positionen und Orientierungen der Molekuele abgebildet, was die Berechnung von Transferintegralen zwischen naechsten Nachbarn mit der Molecular-Orbital-Overlap-Methode ermoeglicht. Dies ergibt realistische Transferintegralverteilungen und dazugehoerige Autokorrelationen. Fuer organische Kristalle untersuchen wir zusaetzlich den Unterschied zwischen zwei Methoden zur Simulation von Ladungstransport, naemlich Semi-Classical-Dynamics (SCD) und Kinetic-Monte-Carlo (KMC) mit Marcusraten. Um den Ladungstransport mit KMC zu simulieren, werden die Schwerpunkte der Molekuele mit Gitterpunkten identifiziert, waehrend die Transferintegrale zur Berechnung der Sprungraten dienen. Im Grenzfall kleiner Polaronen (SCD), indem die Wellenfunktion der Elektronen ueber mehrere Molekuele ausgedehnt ist, wird die Schroedingergleichung semi-klassisch geloest. Die Ergebnisse fuer die unterschiedlichen Materialien und Methoden werden verglichen, sofern vorhanden auch mit experimentellen Daten. Die Carbazolringe bilden hexagonal angeordnete Saeulen, deren Seitenketten nach innen gerichtet sind, was eine dichte Annaeherung und somit interkolumnaren Ladungstransport ermoeglicht. Beruecksichtigt man nur intrakolumnaren Transport, ist die Mobilitaet um mehrere Groeßenordnungen niedriger als im dreidimensionalen Fall. BTBT ist ein vielversprechendes Material fuer in Loesung hergestellte Feldeffekttransistoren. Wir zeigen, dass auf der Zeitskala von Ladungstransport statische Unordnung aufgrund langsam fluktuierender Seitenketten die Mobilitaet erheblich beeinflusst. Die resultierenden Transferintegralverteilungen modifizieren die Konnektivitaet des Systems, jedoch verbleiben genug Perkolationswege fuer schnellen Ladungstransport. Rubren, Indolocarbazol und BBBT sind Kristalle, in denen statische Unordnung eine untergeordnete Rolle spielt. Die hohe Mobilitaet in Rubren wird durch zwei Eigenschaften erklaert: erstens die planare, verschobene Anordnung der Molekuele, und zweitens deren hohe Schwingungsfrequenz. Im Vergleich zu SCD ist KMC basierend auf Marcusraten in der Lage, Nachbarn mit niedrigen Transferraten und dreidimensionale statische Unordnung zu beschreiben. Diese Methode ist daher fuer Systeme, die von statischer Unordnung bestimmt werden, zu bevorzugen. Allerdings ist sie ungeeignet fuer den Fall hoher Transferraten und unterschaetzt die Mobilitaet in gut geordneten Kristallen. SCD ist zwar eindimensional, aber dennoch gut geeignet, um stark gekoppelte Materialien mit wenig Unordnung zu beschreiben. Zusaetzlich ermoeglicht es die Beschreibung dynamischer Effekte, wie intermolekularer Schwingungen. Ratenbasierende Methoden sind dazu nicht in der Lage, da sie auf statischen Momentaufnahmen basieren. Wir haben damit die Vor- und Nachteile zweier aktueller Modelle zur Untersuchung von Ladungstransport in organischen Molekuelen gezeigt, an einem Programm zur Berechnung und Visualisierung von Transferintegralen und Transporteigenschaften mitgearbeitet und Zusammenhaenge zwischen Struktur und Eigenschaften vielversprechnder organischer Halbleiter gefunden

Theoretical methods for studying charge and spin separation in excited states of large molecules and condensed phase

In recent years the GW/BSE approach as a sophisticated many-body method gained considerable attention for ab-initio calculations of a range of properties in finite and infinite systems. For instance, several benchmarks exist for ionization potentials, electron affinities, (band) gaps, and electronically excited states demonstrating an overall good performance of the GW/BSE approach at a computational cost comparable to time-dependent density functional theory (TD-DFT) which is a widely applied method in quantum chemistry. The GW/BSE method outperforms TD-DFT for accurate description of charge-transfer states due to explicit capture of non-local electron-hole interaction mediated by the screened Coulomb potential $W(r,r^{'},omega)$. Furthermore, dynamical correlation is properly described through explicit frequency dependency of $W(r,r^{'},omega)$. Long-range dispersion effects are accounted for by infinite summation of non-local electron correlation contributions; the so-called ring diagrams within the random-phase approximation (RPA). Therefore, the GW/BSE method provides a reliable theoretical tool with a satisfactory prediction power for electronic and optical properties of materials at different phases, and hence is consistently used in this thesis for different types of problems. In the first part of this thesis, the effect of electron-electron correlation, electron-phonon coupling and vertex corrections on the electronic band structure of ice and liquid water within the many-body Green's function formalism (the GW method) is investigated. Furthermore, within the same methodology and based on the Bethe-Salpeter equation (BSE) linear optical absorption spectra of antiferromagnetic zinc ferrite, water and ammonia in the condensed phase are calculated and analyzed in detail. Here, the electron-hole correlation which is responsible for the observed red-shift of absorption peaks and spectral weight redistributions is explicitly taken into account. The electron-hole effects are also of extreme importance for the non-linear absorption spectrum of liquid water (two-photon spectrum) in combination with quasi-particle (QP) effects. The good performance of the GW/BSE methodology is also shown on large donor-acceptor-type molecules, demonstrating its reliability for finite systems where the screening effects are much lower than in periodic systems and a correct description of the long-range behaviour of the exchange-correlation functional is essential. In order to enhance the predictive power of the GW/BSE theory for molecular systems starting from self-interaction free orbitals, a many-body based screening mixing scheme is introduced which remarkably improves the agreement of calculated excitation energies with reference data. In the second part, non-adiabatic excited-state dynamics of condensed water is studied. A combination of ab-initio Born-Oppenheimer molecular dynamics and time-dependent density functional theory is applied. The complex proton dynamics is investigated by large-scale excited-state calculations. It is found that instantaneous concerted hops of protons to the neighboring water molecules (Grotthuss mechanism) are highly unlikely. Furthermore, the solvated electron formed upon proton transfer in the excited state is not fully localized within a cavity-like environment as a consequence of attractive interaction with the surrounding water molecules