1,720,982 research outputs found
Geometry optimization within a localized CAS-SCF approach
No full textA CAS-SCF algorithm based on molecular orbitals that conserve their physical nature during the iterative process has been recently proposed. The algorithm is based on the iterative partial diagonalization of the one-body density matrix obtained from a SuperCI-like procedure. If localized guess orbitals are used, the locality property is conserved by the final orbitals, and the algorithm is particularly suitable for the study of local processes (e.g., bond breaking) in large molecules. In this work, it is shown that the localized orbitals obtained in such a way can be supplied to a standard CAS-SCF geometry-optimization package, in order to find the optimal geometry relaxation with a given local active space. The procedure is illustrated in the case of the rotation of the CH2 group around the C=C double bond in the acroleine molecule
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
Introduction of n-electron valence states for multireference perturbation theory
No full textThe present work presents three second-order perturbative developments from a complete active space (CAS) zero-order wave function, which are strictly additive with respect to molecular dissociation and intruder state free. They differ by the degree of contraction of the outer-space perturbers. Two types of zero-order Hamiltonians are proposed, both are bielectronic, incorporating the interactions between electrons in the active orbitals, therefore introducing a rational balance between the zero-order wave function and the outer-space. The use of Dyall's Hamiltonian, which puts the active electrons in a fixed core field, and of a partially contracted formalism seems a promising compromise. The formalism is generalizable to multireference spaces which are parts of a CAS. A few test applications of the simplest variant developed in this paper illustrate its potentialitie
A novel intermolecular potential to describe the interaction between the azide anion and carbon nanotubes
No full textIn this contribution we propose a novel and accurate intermolecular potential that can be used for the simulation of the azide anion confined inside carbon nanotubes of arbitrary size. The peculiarity of our approach is to include an explicit term, modeling the induction attractive contributions from the negatively charged azide ion, that can be generalized to other ions confined in carbon nanotubes of different size and length. Through a series of accurate DLPNO-CCSD(T) calculations, we show that this potential reproduces the ab initio interaction energy to within a few kcal/mol. The potential is implemented in a molecular dynamics program, with which we carried out illustrative simulations to demonstrate the effectiveness of our approach. At last, the guidelines provided by this investigation can be applied to build up force fields for many neutral/ionic molecular species confined within carbon nanotubes; a crucial requirement to carry out molecular dynamics simulations under a variety of conditions
Toward a Generalized Hückel Rule: The Electronic Structure of Carbon Nanocones
No full textIn this work, we investigate a particular class of carbon nanocones, which we name graphannulenes, and present a generalized Hückel rule (GHR) that predicts the character of their ground state based on simply the three topological indices that uniquely define them. Importantly, this rule applies to both flat and curved systems, encompassing a wide variety of known structures that do not satisfy the "classic"4n + 2 rule such as coronene, corannulene, and Kekulene. We test this rule at the Hückel level of theory for a large number of systems, including structures that are convex and flat, with a saddle-like geometry, and at the CASSCF level of theory for a selected representative subset. All the performed calculations support the GHR that we propose in this work
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