1,721,019 research outputs found
Electronic excitation energies of molecules in solution within continuum solvation models: Investigating the discrepancy between state specific and linear response methods
No full textIn a recent article (R. Cammi, S. Corni, B. Mennucci, and J. Tomasi, J. Chem. Phys. 122, 104513,2005), we demonstrated that the state-specific (SS) and the linear-response (LR) approaches, two different ways to calculate solute excitation energies in the framework of quantum-mechanical continuum models of solvation, give different excitation energy expressions. In particular, they differ in the terms related to the electronic response of the solvent. In the present work, we further investigate this difference by comparing the excitation energy expressions of SS and LR with those obtained through a simple model for solute-solvent systems that bypasses one of the basic assumptions of continuum solvation models, i.e., the use of a single Hartree product of a solute and a solvent wave function to describe the total solute-solvent wave function. In particular, we consider the total solute-solvent wave function as a linear combination of the four products of two solute states and two solvent electronic states. To maximize the comparability with quantum-mechanical continuum model the resulting excitation energy expression is recast in terms of response functions of the solvent and quantities proper for the solvated molecule. The comparison of the presented expressions with the LR and SS ones enlightens the physical meaning of the terms included or neglected by these approaches and shows that SS agrees with the results of the four-level model, while LR includes a term classified as dispersion in previous treatments and neglects another related to electrostatic. A discussion on the possible origin of the LR flaw is finally given
On the calculation of infrared intensities in solution within the Polarizable Continuum Model
No full textWe present a methodology for the theoretical evaluation of infrared intensities for molecules in solution in the polarizable continuum model (PCM) framework. In particular we focus on the calculation of terms related to the solvent polarization induced by the probing field (cavity field term) and on their dependence on the cavity geometry. Numerical tests for few model molecules have been done and compared with semiclassical models
First-principles DFT calculations of Electron Transfer rates in Azurin Dimers
No full textWe have conceived and implemented a new method to calculate transfer integrals between molecular sites, which exploits few quantities derived from density-functional theory electronic structure computations and does not require the knowledge of the exact transition state coordinate. The method uses a complete multielectron scheme, thus including electronic relaxation effects. Moreover, it makes no use of empirical parameters. The computed electronic couplings can then be combined with estimates of the reorganization energy to evaluate electron-transfer rates that are measured in kinetic experiments: the latter are the basis to interpret electron-transfer mechanisms. We have applied our approach to the study of the electron self-exchange reaction of azurin, an electron-transfer protein belonging to the family of cupredoxins. The transfer integral estimates provided by the proposed method have been compared with those resulting from other computational techniques, from empirical models, and with available experimental data
Unravelling single metalloprotein electron transfer by scanning probe techniques
No full textThis review is intended to account for the experimental and theoretical achievements obtained in a period of about 15 years on the investigation of the electron transport through single redoxmetalloproteins by scanning probe techniques. A highly focussed research effort has been deployed by the scientists active in this particular field towards measuring and interpreting electronic current signals flowing via blue copper, redox metalloproteins (e.g. azurin). The field has taken a remarkable advantage of the use of electrochemically assisted scanning tunnelling microscope (EC-STM) which has allowed to probe single molecule signals under full control of all the potential values involved in the experiments. This experimental activity has both triggeredmore comprehensive theoretical interpretations and has been, in its turn, stimulated by theoreticians to test always new predictions. The authors hope to have succeeded in providing the reader with a valuable appraisal of this fascinating field
The cavity electromagnetic field within the polarizable continuum model of solvation: An application to the real-time time dependent density functional theory
No full textWe present an extension of the the cavity-field formulation for the Polarizable Continuum Model (PCM), to the Real-Time Time Dependent Density Functional Theory (RT-TDDFT). Both the length- and velocity-gauge formalisms are developed, through the approach based on effective dipole moment and momentum operators. We apply our formulation to the calculation of imaginary parts of the electric and mixed (electric-magnetic) polarizabilities, i.e. transition dipole moments and rotatory strengths for a model system, the twisted ethylene. To validate our approach we first compare the results obtained with the corresponding properties calculated within a PCM Linear-Response (LR) TDDFT formalism, and we check numerically the gauge-invariance of our formulation. Finally we also compare with the results of analytical models
Nonplasmonic Metal Particles as Excitation Energy Transfer Acceptors: an Unexpected Efficiency Revealed by Quantum Mechanics
No full textWe present a time-dependent density functional theory investigation of the excitation energy transfer (EET) between a typical chromophore (perylene diimide) and a small metal cluster (Au20). Two different physical
descriptions are tested for the metal: the real (nonplasmonic) cluster and a hypothetical nanoparticle characterized by a bulk gold dielectric response (which thus sustains surface plasmons). By comparing the results obtained for the same EET process with the two types of metal particles, we show that, surprisingly, nonplasmonic small metal clusters can be as intrinsically effective as plasmonic particles in EET, a finding
that is also relevant for applications in several field ranging from analytical chemistry to nano science
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
- …
