1,720,983 research outputs found
A priori complete active space self consistent field localized orbitals: an application on linear polyenes
No full textA recently proposed a priori localization technique is used to exploit the possibility to reduce the number of active orbitals in a Complete Active Space Self Consistent Field calculation. The work relies on the fact that the new approach allows a strict control on the nature of the active orbitals and therefore makes it possible to include in the active space only the relevant orbitals. The idea is tested on the calculation of the energy barrier for rigid rotation of linear polyenes. In order to obtain a relevant set of data, a number of possible rotations around double bonds have been considered in the ethylene, butadiene, hexatriene, octatetraene, decapentaene, dodecahexaene molecules. The possibility to reduce the dimension of the active space has been investigated, considering for each possible rotation different active spaces ranging from the minimal dimension of 2 electrons in 2 pi orbitals to the pi-complete space. The results show that the rigid isomerization in the polyene molecules can be described with a negligible loss in accuracy with active spaces no larger than ten orbitals and ten electrons. In the special case of the rotation around the terminal double bond, the space can be further reduced to six orbitals and six electrons with a large decrease of the computational cost. An interesting summation rule has been found and verified for the stabilization of the energy barriers as a function of the dimension of the conjugated lateral chains and of the dimension of the active space
Toward Accurate QM/MM Reaction Barriers with Large QM Regions Using Domain Based Pair Natural Orbital Coupled Cluster Theory
No full textThe hydroxylation reaction catalyzed by p-hydroxybenzoate hydroxylase and the Baeyer-Villiger reaction catalyzed by cyclohexanone monooxygenase are investigated by means of quantum mechanical/molecular mechanical (QM/MM) calculations at different levels of QM theory. The geometries of the stationary points along the reaction profile are obtained from QM/MM geometry optimizations, in which the QM region is treated by density functional theory (DFT). Relative energies are determined from single-point QM/MM calculations using the domain-based local pair natural orbital coupled cluster DLPNO-CCSD(T) method as QM component. The results are compared with single-point DFT/MM energies obtained using popular density functionals and with available experimental and computational data. It is found that the choice of the QM method strongly affects the computed energy profiles for these reactions. Different density functionals provide qualitatively different energy barriers (variations of the order of 10 kcal/mol in both reactions), thus limiting the confidence in DFT/MM computational predictions of energy profiles. On the other hand, the use of the DLPNO-CCSD(T) method in conjunction with large QM regions and basis sets makes it possible to achieve high accuracy. A critical discussion of all the technical aspects of the calculations is given with the aim of aiding computational chemists in the application of the DLPNO-CCSD(T) methodology in QM/MM calculations
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
Decomposition of Intermolecular Interaction Energies within the Local Pair Natural Orbital Coupled Cluster Framework
No full textThe local coupled cluster method DLPNO-CCSD(T) allows calculations on systems containing hundreds of atoms to be performed while typically reproducing canonical CCSD(T) energies with chemical accuracy. In this work, we present a scheme for decomposing the DLPNO-CCSD(T) interaction energy between two molecules into physical meaningful contributions, providing a quantification of the most important components of the chemical interaction. The method, called Local Energy Decomposition (LED), is straightforward and requires negligible additional computing time. Both the Hartree-Fock and the correlation energy are decomposed into contributions from localized or pairs of localized occupied orbitals. Assigning these localized orbitals to fragments allows one to differentiate between intra- and intermolecular contributions to the interaction energy. Accordingly, the interaction energy can be decomposed into electronic promotion, electrostatic, exchange, dynamic charge polarization, and dispersion contributions. The LED scheme is applied to a number of test cases ranging from weakly, dispersively bound complexes to systems with strong ionic interactions. The dependence of the results on the one-particle basis set and various technical aspects, such as the localization scheme, are carefully studied in order to ensure that the results do not suffer from technical artifacts. A numerical comparison between the DLPNO-CCSD(T)/LED and the popular symmetry adapted perturbation theory (DFT-SAPT) is made, and the limitations of the proposed scheme are discussed
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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