1,720,960 research outputs found
Kinetics and dynamics for chemical reactions in gas phase
Full text linkA deep understanding of molecular reactions is a challenging task since the range of time and energy covered implies a wide and dense grid for the numerical representation of the reactive Hamiltonian.
For a computational chemist, the accurate prediction of its value starting from the definition of reactants and products is fascinating and demanding, but can be extremely useful for further investigation and optimization problems.
Several methods, all derived by the Transition State Theory, have been developed to avoid the computational cost of the Hamiltonian representation on a large, multidimensional grid; we investigate these strategies both in the time and energy domain to explore the advan- tages and drawbacks of these reciprocal spaces.
Since we want to increase the range of applicability of the calcula- tion of thermal rate constants to medium size molecules, which can have floppy geometries with low frequency modes, we introduce a dedicated treatment of such modes based on the Intrinsic Reaction Path of Fukui.
In Part i, we introduce the theoretical instrument used to perform our calculation, both in energy and time domain; Part ii is devoted to the presentation of the applications, mainly focused on current issues in astrochemical studies. Appendices treat specific topics, like Möller operators, essential for the comprehension of the theory but too long to be inserted in Part i
Methods for Calculating Partition Functions of Molecules Involving Large Amplitude and/or Anharmonic Motions
Full text linkAnalysis of Partition Functions for Metallocenes: Ferrocene, Ruthenocene, and Osmocene
No full textWe present a calculation of the torsional potential of the three metallocenes of the iron group, that is, ferrocene, ruthenocene, and osmocene, calculated with the GAUSSIAN program suite. Both a variational method (through computation of the exact energy levels) and our Chebyshev imaginary time propagation method are used to calculate the hindered rotation partition function, demonstrating the efficiency of the Chebyshev scheme. The transition from a semirigid through a hindered rotor to the free rotor regime is demonstrated, and the effect of the hindered rotation (as opposed to a harmonic) treatment on the thermodynamics of metallocenes is demonstrated
State-of-the-Art Thermochemical and Kinetic Computations for Astrochemical Complex Organic Molecules: Formamide Formation in Cold Interstellar Clouds as a Case Study
No full textWe describe an integrated
computational strategy aimed at providing
reliable thermochemical and kinetic information on the formation processes
of astrochemical complex organic molecules. The approach involves
state-of-the-art quantum-mechanical computations, second-order vibrational
perturbation theory, and kinetic models based on capture and transition
state theory together with the master equation approach. Notably,
tunneling, quantum reflection, and leading anharmonic contributions
are accounted for in our model. Formamide has been selected as a case
study in view of its interest as a precursor in the abiotic amino
acid synthesis. After validation of the level of theory chosen for
describing the potential energy surface, we have investigated several
pathways of the OH + CH2NH and NH2 + H2CO reaction channels. Our results show that both reaction channels
are essentially barrierless (in the sense that all relevant transition
states lie below or only marginally above the reactants) and once
tunneling is taken into the proper account indicate that the reaction
can occur under the low temperature conditions of interstellar environments
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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