Entanglement entropy of electronic excitations

© 2016 Author(s). A new perspective into correlation effects in electronically excited states is provided through quantum information theory. The entanglement between the electron and hole quasiparticles is examined, and it is shown that the related entanglement entropy can be computed from the eigenvalue spectrum of the well-known natural transition orbital (NTO) decomposition. Non-vanishing entanglement is obtained whenever more than one NTO pair is involved, i.e., in the case of a multiconfigurational or collective excitation. An important implication is that in the case of entanglement it is not possible to gain a complete description of the state character from the orbitals alone, but more specific analysis methods are required to decode the mutual information between the electron and hole. Moreover, the newly introduced number of entangled states is an important property by itself giving information about excitonic structure. The utility of the formalism is illustrated in the cases of the excited states of two interacting ethylene molecules, the conjugated polymer para-phenylene vinylene, and the naphthalene molecule

Visualisation of electronic excited-state correlation in real space

A method for the visualisation of excited‐state electron correlation is introduced and shown to address two notorious problems in excited‐ state electronic structure theory, the analysis of excitonic correlation and the distinction between covalent and ionic wavefunction character. The method operates by representing the excited state in terms of electron and hole quasiparticles, fixing the hole on a fragment of the system and observing the resulting conditional electron density in real space. The application of this approach to oligothiophene, an exemplary conjugated polymer, illuminates excitonic correlation effects of its excited states in unprecedented clarity and detail. A study of naphthalene shows that the distinction between the ionic and covalent states of this molecule, which has so far only been achieved using elaborate valence‐bond theory protocols, arises naturally in terms of electron‐hole avoidance and enhanced overlap, respectively. More generally, the method is relevant for any excited state that cannot be described by a single electronic configuration

TheoDORE: a toolbox for a detailed and automated analysis of electronic excited state computations

The advent of ever more powerful excited-state electronic structure methods has lead to a tremendous increase in the predictive power of computation but it has also rendered the analysis of these computations more and more challenging and time-consuming. TheoDORE tackles this problem through providing tools for post-processing excited-state computations, which automate repetitive tasks and provide rigorous and reproducible descriptors. Interfaces are available for ten different quantum chemistry codes and a range of excited-state methods implemented therein. This article provides an overview of three popular functionalities within TheoDORE, a fragment-based analysis for assigning state character, the computation of exciton sizes for measuring charge transfer, and the natural transition orbitals used not only for visualisation but also for quantifying multiconfigurational character. Using the examples of an organic push-pull chromophore and a transition metal complex, it is shown how these tools can be used for a rigorous and automated assignment of excited-state character. In the case of a conjugated polymer, we venture beyond the limits of the traditional molecular orbital picture to uncover spatial correlation effects using electron-hole correlation plots and conditional densitie

Dynamics simulation of excited state intramolecular proton transfer

Der doppelte Protonentransfer von [2,2'-bipyridyl-]-3,3'-diol im angeregten Zustand wurde mit Hilfe von ab-initio Molekulardynamik simuliert. Die zeitabhängigen experimentellen Spektren konnten gut reproduziert werden. Die Simulation erlaubte es, den Prozess direkt zu beobachten und das Verständnis der Reaktion zu verfeinern. Insbesondere wurde beobachtet, dass das System dynamischer ist als bisher angenommen und keine zwei getrennten Reaktionswege für Einfach- und Doppeltransfer vorliegen.The excited state double proton transfer of [2,2'-bipyridyl-]-3,3'-diol was simulated with ab-initio molecular dynamics simulations. Through this the time-dependent experimental spectra could be well reproduced. The simulation gave the possibility to directly observe the process and improve the understanding of this reaction. Particularly it was observed that the system is highly dynamical and that there are not two separate reaction branches for single and double transfer

Communication: Unambiguous comparison of many-electron wavefunctions through their overlaps

© 2016 Author(s). A simple and powerful method for comparing many-electron wavefunctions constructed at different levels of theory is presented. By using wavefunction overlaps, it is possible to analyze the effects of varying wavefunction models, molecular orbitals, and one-electron basis sets. The computation of wavefunction overlaps eliminates the inherent ambiguity connected to more rudimentary wavefunction analysis protocols, such as visualization of orbitals or comparing selected physical observables. Instead, wavefunction overlaps allow processing the many-electron wavefunctions in their full inherent complexity. The presented method is particularly effective for excited state calculations as it allows for automatic monitoring of changes in the ordering of the excited states. A numerical demonstration based on multireference computations of two test systems, the selenoacrolein molecule and an iridium complex, is presented

Quantum Mechanical Simulations of Defect Dynamics in DNA and Model Systems

In dieser Arbeit wurde die Dynamik von elektronischen Defekten untersucht, die in DNA durch die Einwirkung von UV Strahlung oder oxidativem Stress entstehen können. Ultraschnelle dynamische Prozesse bestimmen, ob diese Defekte zu Schäden in der DNA führen oder ohne negative Effekte deaktiviert werden können. Die Simulation dieser Dynamik stellt wegen der ausgedehnten Struktur von DNA, wegen des Einflusses von nicht-adiabatischen Effekten, und wegen komplexer offenschaliger Wellenfunktionen der Elektronen einige Herausforderungen dar. Um all diese Phänomene zuverlässig beschreiben zu können, wurde ein systematischer Zugang gewählt, der von kleineren Modellsystemen zu realistischen DNA Fragmenten führte. Durch Simulationen von Ladungstransfer Dynamik an kleineren Modellsystemen konnte der Einfluss von nicht-adiabatischen Effekten und Polarisation der Umgebung untersucht werden. In dieser Phase konnte schon viel interessante Einsicht in die zugrunde liegenden physikalischen Phänomene gewonnen werden. Darüberhinaus konnte die Anwendbarkeit von verschiedenen methodischen Strategien untersucht werden, wobei einige kritische Punkte identifiziert wurden. Ausgiebige Methodenentwicklung in Bezug auf SA-MCSCF Gradienten, lokale Diabatisierung für Surface Hopping Dynamik und QM/MM Dynamik wurde durchgeführt um diese Probleme zu behandeln und verlässliche Simulationen an größeren Systemen zu ermöglichen. Zusätzlich wurde eine Analyse von angeregten Zuständen in Systemen mit mehreren gekoppelten Chromophoren ausgearbeitet und implementiert. Durch Rechnungen an größeren Systemen konnte Einsicht in verschiedene komplexe Phänomene gewonnen werden. Zuerst wurden die Prozesse, die in pi-stacks zur Bildung eines Excimers führen untersucht. Dabei konnte vor allem gezeigt werden, dass ein stark stabilisierter kohärenter Zustand durch Wechselwirkungen von Ladungstransfer- und excitonischen Konfigurationen entsteht. Weiters wurden in einem Modellsystem für Wasserstoff gebundene DNA Basenpaare Energietransfer und Protonen gekoppelte Elektronentransfer Prozesse untersucht, wobei vor allem der Einfluss von nicht-adiabatischen Effekten aufgezeigt werden konnte. Schließlich wurden realistische Simulationen der absorbierenden Zustände in DNA durchgeführt. Diese eröffnen neue Perspektiven zu verschiedenen Fragen bezüglich deren excitonischem und Ladungstransfer Charakter, die bisher in der Literatur kontrovers diskutiert wurden.The purpose of this study was the examination of the dynamics of electronic defects introduced into DNA by UV irradiation or oxidative stress. Such defects may either lead to damage in the DNA structure or to deactivation without adverse effects, as determined by ultrafast processes. The simulation of such dynamics presents several challenges, related to the extended structure of DNA, the presence of non-adiabatic interactions between electronic and nuclear degrees of freedom, and complex open-shell electronic wavefunctions requiring accurate computation and a meaningful analysis. To allow for a reliable description of all these phenomena a systematic approach going from smaller model systems to realistic DNA fragments was chosen. Simulations of charge transfer dynamics on smaller systems were performed to give a detailed examination of the influence of non-adiabatic effects and of environmental polarization. Already at this stage very interesting insight into the underlying physics of such processes could be obtained. Moreover, the applicability of the available methodologies could be assessed and several critical points were identified. Significant method development, concerned with state-averaged MCSCF gradients, with a local diabatization method for surface hopping dynamics, and with QM/MM simulations, was performed to address these points leading the way to more extended simulations. In addition an analysis procedure for excited states in systems with several coupled chromophores was devised and implemented. By considering larger systems new insight into several complex phenomena could be obtained. Firstly, the processes leading to excimer formation in stacked pi-systems were examined in the naphthalene dimer. It was found that a strongly stabilized coherent excited state was formed through interactions between excitonic and CT configurations. Secondly, excitation energy transfer as well as proton coupled electron transfer in a hydrogen bonded base-pair analog were analyzed highlighting in particular non-adiabatic effects, which are involved in these processes. Finally, a realistic simulation of the absorbing states in DNA was performed, shedding new light onto several questions regarding excitonic and charge transfer character of these states that had been discussed controversially in literature

Uv absorption inmetal decorated boron nitride flakes: A theoretical analysis of excited states

© Informa UK Limited, trading as Taylor & Francis Group. The excited states of singlemetal atom(X=Co, Al and Cu) doped boron nitride flake (MBNF) B 15 N 14 H 14 -X and pristine boron nitride (B 15 N 15 H 14 ) are studied by time-dependent density functional theory. The immediate effect of metal doping is a red shift of the onset of absorption from about 220 nmfor pristine BNF to above 300 nm for all metal-doped variants with the biggest effect for MBNF-Co, which shows appreciable intensity even above 400 nm. These energy shifts are analysed by detailed wavefunction analysis protocols using visualisationmethods, such as the natural transition orbital analysis and electron-hole correlation plots, as well as quantitative analysis of the exciton size and electronhole populations. The analysis shows that the Co and Cu atoms provide strong contributions to the relevant states whereas the aluminium atom is only involved to a lesser extent

Quantitative wave function analysis for excited states of transition metal complexes

The character of an electronically excited state is one of the most important descriptors employed to discuss the photophysics and photochemistry of transition metal complexes. In transition metal complexes, the interaction between the metal and the different ligands gives rise to a rich variety of excited states, including metal-centered, intra-ligand, metal-to-ligand charge transfer, ligand-to-metal charge transfer, and ligand-to-ligand charge transfer states. Most often, these excited states are identified by considering the most important wave function excitation coefficients and inspecting visually the involved orbitals. This procedure is tedious, subjective, and imprecise. Instead, automatic and quantitative techniques for excited-state characterization are desirable. In this contribution we review the concept of charge transfer numbers---as implemented in the TheoDORE package---and show its wide applicability to characterize the excited states of transition metal complexes. Charge transfer numbers are a formal way to analyze an excited state in terms of electron transitions between groups of atoms based only on the well-defined transition density matrix. Its advantages are many: it can be fully automatized for many excited states, is objective and reproducible, and provides quantitative data useful for the discussion of trends or patterns. We also introduce a formalism for spin-orbit-mixed states and a method for statistical analysis of charge transfer numbers. The potential of this technique is demonstrated for a number of prototypical transition metal complexes containing Ir, Ru, and Re. Topics discussed include orbital delocalization between metal and carbonyl ligands, nonradiative decay through metal-centered states, effect of spin-orbit couplings on state character, and comparison among results obtained from different electronic structure methods.Comment: 47 pages, 19 figures, including supporting information (7 pages, 1 figure

Polyradical Character of Triangular Non-Kekulé Structures, Zethrenes, p -Quinodimethane-Linked Bisphenalenyl, and the Clar Goblet in Comparison: An Extended Multireference Study

In this work, two different classes of polyaromatic hydrocarbon (PAH) systems have been investigated in order to characterize the amount of polyradical character and to localize the specific regions of chemical reactivity: (a) the non-Kekulé triangular structures phenalenyl, triangulene and a π-extended triangulene system with high-spin ground state and (b) PAHs based on zethrenes, p-quinodimethane-linked bisphenalenyl, and the Clar goblet containing varying polyradical character in their singlet ground state. The first class of structures already have open-shell character because of their high-spin ground state, which follows from the bonding pattern, whereas for the second class the open-shell character is generated either because of the competition between the closed-shell quinoid Kekulé and the open-shell singlet biradical resonance structures or the topology of the π-electron arrangement of the non-Kekulé form. High-level ab initio calculations based on multireference theory have been carried out to compute singlet–triplet splitting for the above-listed compounds and to provide insight into their chemical reactivity based on the polyradical character by means of unpaired densities. Unrestricted density functional theory and Hartree–Fock calculations have been performed for comparison also in order to obtain better insight into their applicability to these types of complicated radical systems

Interstate Vibronic Coupling Constants Between Electronic Excited States for Complex Molecules

In the construction of diabatic vibronic Hamiltonians for quantum dynamics in the excited-state manifold of molecules, the coupling constants are often extracted solely from information on the excited-state energies. Here, a new protocol is applied to get access to the interstate vibronic coupling constants at the time-dependent density functional theory level through the overlap integrals between excited-state adiabatic auxiliary wavefunctions. We discuss the advantages of such method and its potential for future applications to address complex systems, in particular those where multiple electronic states are energetically closely lying and interact. As examples, we apply the protocol to the study of prototype rhenium carbonyl complexes [Re(CO)$_3$(N,N)(L)]$^{n+}$ for which non-adiabatic quantum dynamics within the linear vibronic coupling model and including spin-orbit coupling have been reported recently.Comment: 36 pages, 7 figures, 4 table