56 research outputs found
Workshop report. Linear-Scaling Ab Initio Calculations: Applications and Future Directions
No full textThe study of properties and of processes in materials, frequently hinges upon understanding phenomena which originate at the atomic level. In such cases the accurate description of the interactions between large numbers of atoms is critical and in turn requires the accurate description of the electrons which play a crucial role in the bonding of atoms into molecules, surfaces and solids. This can only be achieved by solving the equations of quantum mechanics. These
equations are too complicated to solve exactly; however their solutions can be approximated by computational techniques. The most accurate ? but also most computationally demanding ? are the “ab initio” techniques which do not use any empirical adjustable parameters. Amongst them, the Density Functional Theory (DFT) formulation of quantum mechanics stands out as an excellent compromise between accuracy and computational efficiency. However, the applicability of ab initio techniques is severely limited by poor scaling: the computational effort needed to perform an ab initio calculation increases with (at least) the third power of the number of atoms, N. This cubic-scaling bottleneck limits the number of atoms we can study to a few hundred at most, even on parallel supercomputers. To overcome this length-scale limitation, a number of researchers worldwide have been pioneering the development of a novel class of ab initio methods with linear-scaling or “Order N” (O(N)) computational cost which nevertheless retain the same high level of accuracy as the conventional approaches. While physically motivated, such methods have proved particularly hard to develop as they introduce highly non-trivial localisation constraints. Nevertheless, many major obstacles have been overcome and a number of O(N)
methods (SIESTA, CONQUEST, ONETEP, etc.) for ground state DFT calculations on systems with a gap (e.g. molecules, semiconductors and insulators) are now available and have reached a state of maturity that allows them to be used to study ”real” materials. The particular focus of this workshop is therefore to look forward to what can be achieved in the next few years. Our aim is twofold: (1) As O(N) methods are currently extending the applicability of DFT calculations
to problems involving biomolecules and nanostructures they are leading to completely new levels of understanding of these systems. This CECAM meeting will give us the opportunity to make an appraisal of such large-scale simulations and their potential to connect more directly to experiments. (2) We also want to examine the options for extending linear-scaling to problems that cannot be treated by ground-state DFT but require other, more complex approaches. These include methods for treating metallic systems, excited states and wavefunction-based theories for including electronic correlation. Finding ways to transform these methods to linear-scaling
cost, and hence extent their applicability to the nano-scale, is the next big challenge that the community of developers of large-scale electronic structure methods is beginning to face. We
hope that this workshop will stimulate these major new O(N) methodological developments by bringing together the leading groups in the development of O(N) DFT methods with the leading
groups in the development of metal and excited-state or wavefunction-based methods. Strong emphasis during the workshop will be given to discussion in order to promote the exchange
of ideas between different communities (Physics, Chemistry, Materials Science, Biochemistry) which are all interested in large-scale applications with ab initio accuracy but are approaching
them from different perspectives
L'édition du Liber secretorum eventuum de Jean de Roquetaillade
No full textChristine Morerod-Fattebert, L'édition du Liber secretorum eventuum de Jean de Roquetaillade, p. 297-304.
Le Liber secretorum eventuum de Jean de Roquetaillade a été transmis par des copies complètes, des extraits parfois commentés, des résumés, une traduction catalane ; les manuscrits datent presque tous du XVe siècle. Leurs copistes ont largement et diversement brouillé le sens du texte : c'est l'occasion de s'interroger sur les attentes de l'usager des textes prophétiques ou sur les intentions, dans ce cas, de notre auteur. Une analyse grammaticale détaillée et une comparaison avec une autre œuvre du même auteur, mieux transmise, permet d'affirmer que Roquetaillade avait voulu donner un texte compréhensible, que l'édition cherche à retrouver.Morerod-Fattebert Christine. L'édition du Liber secretorum eventuum de Jean de Roquetaillade. In: Mélanges de l'École française de Rome. Moyen-Age, tome 102, n°2. 1990. Les textes prophétiques et la prophétie en Occident (XII-XVI siècle) pp. 297-304
Une méthode numérique pour la résolution des problèmes aux valeurs propres liés au calcul de structure électronique moléculaire
No full textJean de Roquetaillade. Liber ostensor quod adesse festinant tempora. Édition critique sous la direction d’André Vauchez par Clémence Thévenaz-Modestin et Christine Morerod-Fattebert, avec la collaboration de Marie-Henriette Jullien de Pommerol, sur la base d’une transcription de Jeanne Bignami-Odier
No full textJugie Pierre. Jean de Roquetaillade. Liber ostensor quod adesse festinant tempora. Édition critique sous la direction d’André Vauchez par Clémence Thévenaz-Modestin et Christine Morerod-Fattebert, avec la collaboration de Marie-Henriette Jullien de Pommerol, sur la base d’une transcription de Jeanne Bignami-Odier. In: Bibliothèque de l'école des chartes. 2007, tome 165, livraison 1. pp. 205-207
Trading fear for food in the Anthropocene: How ungulates cope with human disturbance in a multi-use, suburban ecosystem
No full textResource distribution, predation risk and disturbance in space and time can affect how animals use their environment. To date few studies have assessed the spatiotemporal trade-off between resource acquisition and avoidance of risks and human disturbance in small protected areas embedded in an urban matrix. A better understanding of the forage-safety trade-off in urban protected areas (UPA) is key to the design of evidence based approaches to deal with the ever-increasing human-wildlife impacts typical of UPA. Herein, we analyzed camera trap data to evaluate how two ungulate species trade fear for food in a 60 km(2) human-dominated UPA without natural predators. We found that wild boar (Sus scrofa) were predominantly active at night, while roe deer (Capreolus capreolus) showed a typical bimodal crepuscular activity pattern. Occupancy analysis indicated that deciduous forest and the presence of high seats for hunting played an important role in determining the space use of wild boar. For roe deer, we found indications that the presence of forest influenced space use, although the null model was retained among the top ranked models. Our results confirm that wild boar and roe deer are able to thrive in heavily human dominated landscapes characterized by intensive recreational use and hunting, such as protected areas embedded in an urban matrix. (c) 2020 Elsevier B.V. All rights reserved.This work makes use of data and/or infrastructure provided by INBO and funded by Research Foundation - Flanders (FWO) as part of the Belgian contribution to LifeWatch. JW is funded by a BOF-mandate at Hasselt University. We are grateful to ANB, Regionaal Landschap Kempen & Maasland, the municipalities of As, Dilsen-Stokkem, Lanaken, Maasmechelen and Zutendaal, the tourist offices of NPHK, hunters and residents to allow us to place camera traps on their property. Further, we thank all students and volunteers that aided in the field or processed and annotated pictures. Finally, we appreciate the insightful feedback we received from the two reviewers that commented on this manuscript.Wevers, J (corresponding author), Hasselt Univ, Ctr Environm Sci, B-3590 Diepenbeek, Belgium.
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A unified electrostatic and cavitation model for first-principles molecular dynamics in solution
No full textThe electrostatic continuum solvent model developed by Fattebert and Gygi is combined with a first-principles formulation of the cavitation energy based on a natural quantum-mechanical definition for the surface of a solute. Despite its simplicity, the cavitation contribution calculated by this approach is found to be in remarkable agreement with that obtained by more complex algorithms relying on a large set of parameters. The model allows for very efficient Car-Parrinello simulations of finite or extended systems in solution, and demonstrates a level of accuracy as good as that of established quantum-chemistry continuum solvent methods. They apply this approach to the study of tetracyanoethylene dimers in dichloromethane, providing valuable structural and dynamical insights on the dimerization phenomenon
A BLOCK RAYLEIGH QUOTIENT ITERATION WITH LOCAL QUADRATIC CONVERGENCE
No full textAbstract. We present an iterative method, based on a block generalization of the Rayleigh Quotient Iteration method, to search for the p lowest eigenpairs of the generalized matrix eigenvalue problem Au = Bu. We prove its local quadratic convergence when B,1 A is symmetric. The benefits of this method are the well-conditioned linear systems produced and the ability to treat multiple or nearly degenerate eigenvalues. Key words. Subspace iteration, Rayleigh Quotient Iteration, Rayleigh-Ritz procedure. AMS subject classifications. 65F15
Thermo4PFM: Facilitating Phase-field simulations of alloys with thermodynamic driving forces
No full text<p>Presentation given at ChiMAD meeting, March 19, 2024, Northwestern University</p>
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