1,721,195 research outputs found
Benchmarking Time-Dependent Density Functional Theory for Excited State Geometries of Organic Molecules in Gas-Phase and in Solution
No full textWe analyze potentials and limits of the Time- Dependent Density Functional Theory (TD-DFT) approach for the determination of excited-state geometries of organic molecules in gas-phase and in solution. Three very popular DFT exchange- correlation functionals, two hybrids (B3LYP and PBE0) and one long-range corrected (CAM-B3LYP), are here investigated, and the results are compared to the correlated RI-CC2 wave function approach. Solvent effects are further analyzed by means of a polarizable continuum model. A total of 15 organic chromophores (including both small molecules and larger push−pull systems) are considered as prototypes of n → π* and π → π* singlet excitations. Our analysis allows to point out specific correlations between the accuracy of the various functionals and the type of excitation and/or the type of chemical bonds involved. We find that while the best ground-state geometries are obtained with PBE0 and B3LYP, CAM- B3LYP yields the most accurate description of electronic and geometrical characteristics of excited states, both in gas-phase and in solution
On the Photophysics of Carotenoids: A Multireference DFT Study of Peridinin
No full textWe present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining the position and character of its low-lying singlet electronic states is investigated using a multireference DFT approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states, as no evidence points to an intermediate S* state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S 1, 2 1A g - and S 2, 1 1Bu +) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S 1 and S 2 states present distinct natures for small BLA values, whereas for larger ones they become more similar in terms of both brightness and dipolar character and their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.</p
Identifying the Hamiltonian structure in linear response theory
No full textWe present a unifying framework for linear response eigenvalue equations that encompasses both
variational Hartree-Fock and Kohn-Sham density functional theory as well as non-variational
coupled-cluster theory. The joint description is rooted in the so-called Hamiltonian structure of the
response kernel matrices, whose properties permit an immediate identification of the well-known
paired eigenvalue spectrum describing a molecule in the isolated state. Recognizing the Hamiltonian
structure underlying the equations further enables a generalization to the case of a polarizableembedded
molecule treated in variational and, in particular, in non-variational theories
Carotenoids and Light-Harvesting: From DFT/MRCI to the Tamm–Dancoff Approximation
No full textCarotenoids are known to play a fundamental
role in photosynthetic
light-harvesting (LH) complexes; however, an accurate quantum-mechanical
description of that is still missing. This is due to the multideterminant
nature of the involved electronic states combined with an extended
conjugation which limits the applicability of many of the most advanced
approaches. In this study, we apply a multireference configuration
interaction extension of density functional theory (DFT/MRCI) to describe
transition energies and densities as well as the corresponding excitonic
couplings, for the three lowest singlet excited states of nine carotenoids
present in three different LH complexes of algae and plants. These
benchmark results are used to find an approximated computational approach,
which could be used to quantitatively reproduce the key quantities
at a reduced computational cost. To this end, we tested the Tamm–Dancoff
approximation (TDA) to time-dependent density functional theory in
combination with different functionals. By analyzing the errors with
respect to DFT/MRCI-TDA results for the full set of electronic properties,
we conclude that TDA-TPSS with small basis sets indeed represents
an effective approach to investigate LH processes that involve carotenoids
Toward Reliable Prediction of the Energy Ladder in Multichromophoric Systems: A Benchmark Study on the FMO Light-Harvesting Complex
No full textWe present an evaluation of the performance of various single-reference QM methods for the prediction of the relative site energies and transition moments of the Q bands in the bacteriochlorophyll a (BChl a) pigments of the Fenna-Matthew-Olson (FMO) complex. We examine the relative merits of ZINDO, CIS, TD-DFT (with the functionals PBE, BLYP, PBE0, B3LYP, and CAM-B3LYP) and RI-CC2 in reproducing the variations across the pigments that occur as a consequence of geometrical and electrostatic effects of the FMO complex by comparison to DFT-BHLYP/MRCI. We find that these pigments are near-multiconfigurational in nature and, thus, constitute critical cases for the RI-CC2 method. The commonly used ZINDO method is fairly reliable for the site energies of the isolated pigments; however, it overestimates the coupling to the environment, thereby leading to variations across the embedded pigments that are too drastic. The overall best performance is provided by the investigated TD-DFT methods, where PBE0 is found to be slightly superior to the other functionals tested.</p
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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