1,720,995 research outputs found
A hierarchy of potential energy surfaces constructed from energies and energy derivatives calculated on grids
No full textn this work we develop and test a methodology for the generation of Born-Oppenheimer potential energy surfaces (PES) for use in vibrational structure calculations. The method relies on the widely used restricted-mode-coupling expansion of the fully coupled potential surface where only up to n or less vibrational coordinates are coupled in the potential. Low-order derivatives of the energy are then used to extrapolate the higher mode-coupling potential terms; derivative information is thus used in a convenient way for the evaluation of higher mode couplings avoiding their explicit calculation on multidimensional grids. The formulation, which is a variant of the popular modified Shepard interpolation, is general for any extrapolation of (n+p)-mode-coupling terms from n-mode couplings and can be applied to the energy or any other molecular property surface for which derivative information is available. The method depends only on analytical parameter-free weight functions that satisfy important limiting conditions and control the contribution from each direction of extrapolation. The procedure has been applied on a representative set of 13 molecules, and its accuracy has been tested using only gradients and using both gradients and Hessians. The results provide evidence for the importance of higher mode couplings and illustrate the cost efficiency of the proposed approach
Using electronic energy derivative information in automated potential energy surface construction for vibrational calculations
No full textThe availability of an accurate representation of the potential energy surface (PES) is an essential prerequisite in an anharmonic vibrational calculation. At the same time, the high dimensionality of the fully coupled PES and the adverse scaling properties with respect to the molecular size make the construction of an accurate PES a computationally demanding task. In the past few years, our group tested and developed a series of tools and techniques aimed at defining computationally efficient, black-box protocols for the construction of PESs for use in vibrational calculations. This includes the definition of an adaptive density-guided approach (ADGA) for the construction of PESs from an automatically generated set of evaluation points. Another separate aspect has been the exploration of the use of derivative information through modified Shepard (MS) interpolation/extrapolation procedures. With this article, we present an assembled machinery where these methods are embedded in an efficient way to provide both a general machinery as well as concrete computational protocols. In this framework we introduce and discuss the accuracy and computational efficiency of two methods, called ADGA[2gx3M] and ADGA[2hx3M], where the ADGA recipe is used (with MS interpolation) to automatically define modest sized grids for up to two-mode couplings, while MS extrapolation based on, respectively, gradients only and gradients and Hessians from the ADGA determined points provides access to sufficiently accurate three-mode couplings. The performance of the resulting potentials is investigated in vibrational coupled cluster (VCC) calculations. Three molecular systems serve as benchmarks: a trisubstituted methane (CHFClBr), methanimine (CH(2)NH), and oxazole (C(3)H(3)NO). Furthermore, methanimine and oxazole are addressed in accurate calculations aiming to reproduce experimental results
Analysis of nondynamic electron correlation in DFT
No full textA comprehensive description of nondynamic correlation is still an open controversy in the Kohn-Sham Density Functional Theory (KS-DFT). The separation of electron correlation effects is strictly related to the shape of the wavefunction, however, because the KS wavefunction does not describe the realistic system, is not possible to analyze the relevance of electron correlation and its predominant type. Nondynamic correlation is significant for systems with a multiconfigurational character. These systems are treated inadequately by the Density Functional Approximations(DFAs). An estimation of nondynamic effects in KS-DFT is therefore of paramount importance. In this work, an analysis of nondynamic correlation in the context of KS-DFT has been performed. Starting from a KS electronic density, we have extracted information to recover the correlated first order reduce density matrix (1-RDM). The 1-RDM has been obtained in a diagonal form by an expansion over a basis of natural orbitals. Appropriate fractional occupation numbers have been computed by two different approaches. All the calculations have been performed on three diradical molecules, namely para-benzyne, meta-benzyne and ortho-benzyne. These systems have been chosen because they show different percentages of nondynamic correlation. In the first implementation, we have fitted a correlated electronic density computed at Variational Monte Carlo (VMC) level by means of a scheme based on the use of a N-representable Generalized Valence Bond (GVB)form. The GVB wavefunction was expanded from KS-DFT localized bonding and antibonding orbitals. This wavefunction, due to its shape, is able to provide a correlated 1-RDM. The corresponding kinetic energy gives closer results to the VMC one than the starting Ts, namely the KS single particle kinetic energy. The second analysis was done through an unrestricted DFT method, which allows the extraction of fractional occupancies. Several DFAs have been considered in order to analyze how nondynamic correlation is treated in different rungs of the Jacob's ladder. The information on nondynamic correlation from these two sets of occupation numbers has been further tested by computing the correlation indicators developed by Ramos-Cordoba and Matito. The occupation numbers calculated with both approaches are then connected to KS restricted single particle orbital energies through the Fermi distribution. For each fit we obtain a fictitious temperature Tf and a chemical potential μ. The results obtained for the global nondynamic indicators appear to be in some cases contradictory, whereas for the local ones there is an interesting agreement among the data. The fitted occupation numbers that best reproduced the reference ones were the GGA. Therefore, by computing the local indicators for this type of functionals is possible to extract information about nondynamic correlation even in restricted DFT calculations. The results of this thesis could be useful for a development of new kind of functionals that use multiconfigurational densities constructed from KS occupied and virtual orbitals
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
An electronic aromaticity index for large rings
No full textWe introduce a new electronic aromaticity index, AV1245, consisting of an average of the 4-center multicenter indices (MCI) along the ring that keeps a positional relationship of 1, 2, 4, 5. AV1245 measures the extent of transferability of the delocalized electrons between bonds 1–2 and 4–5, which is expected to be large in conjugated circuits and, therefore, in aromatic molecules. A new algorithm for the calculation of MCI for large rings is also introduced and used to produce the data for the calibration of the new aromaticity index. AV1245 does not rely on reference values, does not suffer from large numerical precision errors, and it does not present any limitation on the nature of atoms, the molecular geometry or the level of calculation. It is a size-extensive measure with a low computational cost that grows linearly with the number of ring members. Therefore, it is especially suitable to study the aromaticity of large molecular rings such as those occurring in belt-shaped Möbius structures or porphyrins. The analysis of AV1245 in free-base and bis-metalated Pd [32]octaphyrins(1,0,1,0,1,0,1,0) completes this study
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