1,721,097 research outputs found
Exploring the ultrafast excited-state dynamics of transition metal complexes with ab-inito multiple spawning
Full text linkSeit ihrer Entstehung im letzten Jahrhundert hat sich die Quantenchemie weitverbreitet, mit der bereits routinemäßig eine Vielzahl von Spektren berechnet werden können. Im Gegensatz dazu ist die Modellierung von ultraschnellen photochemischen Prozessen heute noch sehr anspruchsvoll und limitiert auf kleine und mittelgroße Moleküle. Die stetig steigende Computerleistung ermöglicht nun die Berechnung großer Übergangsmetallkomplexe. Aufgrund ihrer hohen Anzahl an elektronischen Zuständen und Atomen bergen diese jedoch einige Herausforderungen für die quantenmechanischen Methoden zur Modellierung von ultraschnellen Prozessen. Daher ist es Ziel dieser Arbeit Methoden zu identifizieren, mit denen diese Herausforderungen bewältigt werden können. Dafür wurde zunächst die Methode ab-initio multiple spawning (AIMS) zur Berechnung der Atomkerndynamik basierend auf einem Vergleich von zwei vielversprechenden Methoden ausgesucht. Anschließend wurde die zeitabhängige Dichtefunktionaltheorie (TDDFT) für die Beschreibung der elektronischen Struktur ausgewählt, da diese Methode einen guten Kompromiss zwischen Genauigkeit und Rechenaufwand ermöglicht. Dabei wurden anhand des Beispielkomplexes Eisenpentacarbonyl auch die Grenzen der Methode aufgezeigt.
Mit den ausgewählten Methoden wurden 500 fs der Dynamik eines Übergangsmetallkomplexes mit mehr als 100 Atomen simuliert, was die bisher größte AIMS Berechnung für einen Übergangsmetallkomplex darstellt. Durch einen Vergleich mit experimentellen Ergebnissen konnte die Simulation validiert werden und weitere Erkenntnisse über den Reaktionsmechanismus des Komplexes konnten gewonnen werden. Außerdem wurde die Dynamik der angeregten Zustände eines experimentell bisher kaum untersuchten Übergangsmetallkomplexes vorhergesagt, um das Potential der hier ausgewählten Methoden aufzuzeigen. Zusammenfassend wurde damit gezeigt, dass die Kombination von AIMS und TDDFT eine effiziente Berechnung der Dynamik von Übergangsmetallkomplexen ermöglicht
Spin-Orbit Coupling with the Auxiliary Many-Electron Wavefunction
No full textSCGCConsultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
When the exact factorization meets conical intersections..
Full text linkCapturing nuclear dynamics through conical intersections is pivotal to understand the fate of photoexcited molecules. The concept of a conical intersection, however, belongs to a specific definition of the electronic states, within a Born–Huang representation of the molecular wavefunction. How would these ultrafast funneling processes be translated if an exact factorization of the molecular wavefunction were to be used? In this article, we build upon our recent analysis [B.F.E. Curchod, F. Agostini, J. Phys. Chem. Lett. 8, 831 (2017)] and address this question in a broader perspective by studying the dynamics of a nuclear wavepacket through two types of conical intersections, differing by the strength of their underlying diabatic coupling. Our results generalize our previous findings by (i) showing that the time-dependent potential energy surface smoothly varies, both in time and in position, between the corresponding diabatic and adiabatic potentials, with sometimes more complex features if interferences are observed, (ii) highlighting the non-trivial behavior of the time-dependent vector potential and the fact that it cannot be gauged away in general, and (iii) justifying some approximations employed in the derivation of a mixed quantum/classical scheme based on the exact factorization.</p
Towards the binding of cisplatin to Cu,Zn superoxide dismutase: First steps of a theoretical study
No full textSCGCGR-53 616; EPFL - SB - ISIC - LCBC; Prix CIBA. Consultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
A Classical and Quantum Trajectory Description of Nonadiabatic Dynamics within Time-Dependent Density Functional Theory
No full textCharacterizing and predicting the nuclear dynamics of electronically excited molecules is of paramount importance to the understanding of photochemical and photophysical processes in molecules and to the development of new technologies in domains like solar energy conversion, efficient illumination devices, and medicine. However, the theoretical description of such phenomena remains a challenge for theoretical chemists. The most notable difficulty comes from the breakdown of the widely used Born-Oppenheimer approximation, which considers the time evolution of the nuclei fully decoupled from the one of the electrons. Moving beyond this approximation requires the inclusion of nonadiabatic effects, which induces an entangled electron-nuclear dynamics. Additionally, the convenient approximation that the motion of the nuclear degrees of freedom can be described with classical mechanics is likely to fail in the case of nonadiabatic events. While several theoretical techniques have been proposed to treat both the electronic structure and the nonadiabatic dynamics problems, they all rely on a compromise between accuracy and efficiency. The principal goal of this thesis is to improve the description of molecular nonadiabatic phenomena using classical and quantum trajectories, combined with linear-response time-dependent density functional theory (LR-TDDFT) for the calculation of the electronic structure properties. In the first part of this work, we review several methods used to solve exactly or in an approximate way the nuclear time-dependent Schrödinger equation with quantum or classical trajectories. We then move on to show how the molecular time-dependent Schrödinger equation can be reformulated exactly in terms of quantum trajectories evolving in coupled adiabatic electronic states. This nonadiabatic Bohmian dynamics (NABDY) scheme allows for the description of all nuclear quantum effects like decoherence and tunneling, and is compatible with a nuclear wavepacket dynamics in which the electronic structure information is computed on-the-fly. Furthermore, we discuss how NABDY can be related, through several approximations, to the trajectory surface hopping (TSH) method, which is one of the most commonly applied on-the-fly trajectory-based techniques to describe the dynamics of molecular systems beyond the Born-Oppenheimer approximation in (the unconstrained) configuration space of molecules. The TSH method describes the nuclear wavepacket dynamics with a swarm of uncorrelated classical trajectories, consequently banishing all nuclear quantum effects. Understanding the underlying limitations of TSH is of foremost importance for the improvement of the theory. In this thesis, several one dimensional model systems are used to assess the accuracy of TSH through a comparison with the correlated NABDY dynamics. In the second part, we focus on the electronic structure problem and discuss how LR-TDDFT can be used in the implementation of an efficient on-the-fly nonadiabatic molecular dynamics scheme. Within this theory, all the electronic information needed, namely excitation energies, excited state nuclear forces, nonadiabatic couplings, and other electronic matrix elements, have to be represented as a functional of the electronic density. We show how the concept of auxiliary many-electron wavefunction can be used to compute the matrix elements of any one-body operators. In the third and last part, we discuss two extensions of the TSH method based on LR-TDDFT, aimed at describing the effects of the environment on a molecular system in the most possible realistic way. First, the effects of an explicit solvent on a photoactive solute is described within a QM/MM formalism. The resulting TSH/LR-TDDFT/MM scheme is applied to the nonradiative relaxation of the inorganic compound ruthenium (II) trisbipyridine in water. This application further highlights the need for the inclusion of relativistic effects in the TSH algorithm such as spin-orbit coupling to describe intersystem crossing processes. Second, the TSH equations are coupled with an external time-dependent electric field, such that photoexcitation processes can be naturally described within this mixed quantum/classical method. The electric field can be either parametrized or shaped to selectively maximize the population of a given target electronic state. TSH dynamics coupled to an electric field is first applied to the study of the photodissociation dynamics of a diatomic molecule (lithium fluoride) and then used for the investigation of the photoinduced proton transfer reaction in an organic compound (4-hydroxyacridine). This thesis presents several possibilities to describe the nonadiabatic dynamics of molecules. In addition, it also highlights some current limitations of both electronic structure and nonadiabatic dynamics methods, proposing new potential solutions to these problems.LCB
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Time-resolved photoelectron imaging of pck anion
No full textraw photoelectron images
analysis of images (photoelectron spectra and anisotropy parameters)
geometries of critical points on S
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