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$_{\mathsf{SU(2)}}$=2500)/cc-pVDZ level of theory. The contribution of $1s$ core correlation to the $X^1\Sigma_g^+$ bond dissociation curve of the carbon dimer was estimated by comparing energies at the DMRG(36o, 12e, D$_{\mathsf{SU(2)}}$=2500)/cc-pCVDZ and DMRG-SCF(34o, 8e, D$_{\mathsf{SU(2)}}$=2500)/cc-pCVDZ levels of theory.Comment: 16 pages, 13 figure

Matrix product states and their uses in quantum chemistry

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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