890 research outputs found
Counting publications and citations: Is more always better?
Is more always better? We address this question in the context of
bibliometric indices that aim to assess the scientific impact of individual
researchers by counting their number of highly cited publications. We propose a
simple model in which the number of citations of a publication depends not only
on the scientific impact of the publication but also on other 'random' factors.
Our model indicates that more need not always be better. It turns out that the
most influential researchers may have a systematically lower performance, in
terms of highly cited publications, than some of their less influential
colleagues. The model also suggests an improved way of counting highly cited
publications
CheMPS2: a free open-source spin-adapted implementation of the density matrix renormalization group for ab initio quantum chemistry
The density matrix renormalization group (DMRG) has become an indispensable
numerical tool to find exact eigenstates of finite-size quantum systems with
strong correlation. In the fields of condensed matter, nuclear structure and
molecular electronic structure, it has significantly extended the system sizes
that can be handled compared to full configuration interaction, without losing
numerical accuracy. For quantum chemistry (QC), the most efficient
implementations of DMRG require the incorporation of particle number, spin and
point group symmetries in the underlying matrix product state (MPS) ansatz, as
well as the use of so-called complementary operators. The symmetries introduce
a sparse block structure in the MPS ansatz and in the intermediary contracted
tensors. If a symmetry is non-abelian, the Wigner-Eckart theorem allows to
factorize a tensor into a Clebsch-Gordan coefficient and a reduced tensor. In
addition, the fermion signs have to be carefully tracked. Because of these
challenges, implementing DMRG efficiently for QC is not straightforward.
Efficient and freely available implementations are therefore highly desired. In
this work we present CheMPS2, our free open-source spin-adapted implementation
of DMRG for ab initio QC. Around CheMPS2, we have implemented the augmented
Hessian Newton-Raphson complete active space self-consistent field method, with
exact Hessian. The bond dissociation curves of the 12 lowest states of the
carbon dimer were obtained at the DMRG(28 orbitals, 12 electrons,
D=2500)/cc-pVDZ level of theory. The contribution of
core correlation to the bond dissociation curve of the carbon
dimer was estimated by comparing energies at the DMRG(36o, 12e,
D=2500)/cc-pCVDZ and DMRG-SCF(34o, 8e,
D=2500)/cc-pCVDZ levels of theory.Comment: 16 pages, 13 figure
Longitudinal static optical properties of hydrogen chains: finite field extrapolations of matrix product state calculations
We have implemented the sweep algorithm for the variational optimization of
SU(2) x U(1) (spin and particle number) invariant matrix product states (MPS)
for general spin and particle number invariant fermionic Hamiltonians. This
class includes non-relativistic quantum chemical systems within the
Born-Oppenheimer approximation. High-accuracy ab-initio finite field results of
the longitudinal static polarizabilities and second hyperpolarizabilities of
one-dimensional hydrogen chains are presented. This allows to assess the
performance of other quantum chemical methods. For small basis sets, MPS
calculations in the saturation regime of the optical response properties can be
performed. These results are extrapolated to the thermodynamic limit.Comment: Submitted to J. Chem. Phy
Trajectories and regimes in research versus knowledge evaluations: Contributions to an evolutionary theory of citation
Citation analysis can provide us with models of the evolutionary dynamics in scholarly and scientific communication. We propose to distinguish between institutional research evaluation (usually, ex post) and knowledge evaluation ex ante, in relation to directionality in citation analysis. We discuss the theoretical literature on communication systems which distinguishes between information and meaning, in which the concept of redundancy plays an important role as measure of the potential of a communication system. This is the basis for a model of knowledge dynamics which differentiates between observable variation and latent selection environments. We use indicators at the journal level and analyze the citation environments of journals in both the cited and citing directions. Among journals, the citing direction can be analyzed by co-citation and indicates the integration of knowledge from different fields. The cited direction can analogously be analyzed by bibliographic coupling and represents the extent to which the cited journal has become relevant for different disciplines, hence indicates knowledge diffusion. We apply this analysis on three different case studies of journal-journal relations: a small scale study of the journal Public Understanding of Science, a random sample of 100 journals, and a large-scale analysis of the set of JCR 2016 journals. Combined, the results seem to confirm the hypothesis that interdisciplinarity cannot be captured by one-dimensional citation analysis. Both citing and cited directions are relevant for knowledge and research evaluations, respectively. We raise the question whether indicators of interdisciplinarity can be developed by combining both directions in citation analysis, indicate further research, and discuss the normative implications of our preliminary results
Assessing the Impact of Publications Saved by Mendeley Users: Is There Any Different Pattern Among Users?
The main focus of this paper is to investigate the impact of publications read (saved) by the different users in Mendeley in order to explore the extent to which their readership counts correlate with their citation indicators. The potential of filtering highly cited papers by Mendeley readerships and its different users have been also explored. For the analysis of the users, we have considered the information of the top three Mendeley ‘users’ reported by the Mendeley. Our results show that publications with Mendeley readerships tend to have higher citation and journal citation scores than publications without readerships. ‘Biomedical & health sciences’ and ‘Mathematics and computer science’ are the fields with respectively the most and the least readership activity in Mendeley. PhD students have the highest density of readerships per publication and Lecturers and Librarians have the lowest across all the different fields. Our precision-recall analysis indicates that in general, for publications with at least one reader in Mendeley, the capacity of readerships of filtering highly cited publications is better than (or at least as good as) Journal Citation Scores. We discuss the important limitation of Mendeley of only reporting the top three readers and not all of them in the potential development of indicators based on Mendeley and its users
Mendeley readership as a filtering tool to identify highly cited publications
This study presents a large scale analysis of the distribution and presence
of Mendeley readership scores over time and across disciplines. We study
whether Mendeley readership scores (RS) can identify highly cited publications
more effectively than journal citation scores (JCS). Web of Science (WoS)
publications with DOIs published during the period 2004-2013 and across 5 major
scientific fields have been analyzed. The main result of this study shows that
readership scores are more effective (in terms of precision/recall values) than
journal citation scores to identify highly cited publications across all fields
of science and publication years. The findings also show that 86.5% of all the
publications are covered by Mendeley and have at least one reader. Also the
share of publications with Mendeley readership scores is increasing from 84% in
2004 to 89% in 2009, and decreasing from 88% in 2010 to 82% in 2013. However,
it is noted that publications from 2010 onwards exhibit on average a higher
density of readership vs. citation scores. This indicates that compared to
citation scores, readership scores are more prevalent for recent publications
and hence they could work as an early indicator of research impact. These
findings highlight the potential and value of Mendeley as a tool for
scientometric purposes and particularly as a relevant tool to identify highly
cited publications
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