12 research outputs found
Langevin dynamics in constant pressure extended systems
No full textThe advantages of performing Langevin dynamics in extended systems are discussed. A simple Langevin dynamics scheme for producing the canonical ensemble is reviewed, and is then extended to the Hoover ensemble. We show that the resulting equations of motion generate the isobaric–isothermal ensemble. The Parrinello–Rahman ensemble is then discussed and we show that despite the presence of intrinsic probability gradients in this system, a Langevin dynamics approach samples the extended phase space in the correct fashion. The implementation of these methods in the ab initio plane wave density functional theory code CASTEP [M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clarke, and M. C. Payne, J. Phys.: Condens. Matter 14, 2717 (2003)] is demonstrated
A periodic genetic algorithm with real-space representation for crystal structure and polymorph prediction
No full textA genetic algorithm is described that is suitable for determining the global minimum energy configurations of crystal structures and which can also be used as a polymorph search technique. This algorithm requires no prior assumptions about unit cell size, shape, or symmetry, nor about the ionic configuration within the unit cell. This therefore enables true ab initio crystal structure and polymorph prediction. Our algorithm uses a real-space representation of the population members, and makes use of a periodic cut for the crossover operation. Results on large Lennard-Jones systems with fcc- and hcp-commensurate cells show robust convergence to the bulk structure from a random initial assignment and an ability to successfully discriminate between competing low enthalpy configurations. Results from an ab initio carbon polymorph search show the spontaneous emergence of both Lonsdaleite and graphite-like structures
Improving the convergence of defect calculations in supercells: An ab initio study of the neutral silicon vacancy
No full textWe present a systematic methodology for the accurate calculation of defect structures in supercells, which we illustrate with a study of the neutral vacancy in silicon. This is a prototypical defect which has been studied extensively using ab initio methods, yet remarkably there is still no consensus about the energy or structure of this defect, or even whether the nearest-neighbor atoms relax inwards or outwards. In this paper, we show that the differences between previous calculations can be attributed to supercell convergence errors, and we demonstrate how to systematically reduce each such source of error. The various sources of scatter in previous theoretical studies are discussed and a different effect, that of supercell symmetry, is identified. It is shown that a consistent treatment of this effect is crucial in understanding the systematic effects of increasing the supercell size. This work therefore also presents the best converged ab initio study of the neutral silicon vacancy to date
A framework towards forward cooperation in the product life cycle of dredging equipment
No full textThe Dutch shipbuilding industry has a real competitive force in building complex, specialized and multipurpose vessels (Rabobank, 2014). However, the Dutch maritime sector has to deal with strongly growing competition worldwide. For the dredging sector this implies the need to innovate in order to remain competitive. This research focuses on the strategy of vertical forward cooperation to do so; the supplier will move into the maintenance aspects of dredging equipment, benefiting both the client and the supplier by expanding their services. This led to the research question of how a supplier of dredging equipment can achieve forward cooperation in the product life cycle. As a case study the company of Royal IHC is used for this research, in specific the product type of the IHC Beaver dredger is discussed. The research is divided into six parts; (i) a literature study; where the possible strategies for the forward cooperation are studied; a case study, where the (ii) IST (current) situation and the (iii) SOLL (ideal) situation are defined according to the current situation at IHC and a benchmark study. A system characterization for warranty cost analysis by D.N.P. Murthy and W.R. Blischke (2006) that describes how warranty policy, product reliability and product usage ultimately influence the warranty costs is used to structure these situations. Then a GAP analysis follows (iv) where the IST and SOLL are compared. This leads to the (v) fifth part of the research: the framework on how to achieve forward cooperation, which is supported with a model. Finally (vi) the conclusions and recommendations are given. Firstly the current situation (IST) is researched in which the IHC Beaver is provided with one-year warranty at sale. Two important groups of stakeholders are the clients and the sub-suppliers. For the procurement of parts the role of the sub-suppliers is essential because they deliver over 60% of the parts of the Beaver dredger. How agreements are made with sub-suppliers on the warranty period of the supplied parts is therefore essential when IHC has the intention to apply forward cooperation in the future. After the IST situation the ideal situation (SOLL) is defined. Forward cooperation can be achieved with different strategies; with the properties of the dredging market and the intention to achieve forward cooperation in the product life cycle of a standard industrial product, extended warranty has the best characteristics. Compared to the IST situation with base-warranty it is concluded that several uncertainties arise when extending the warranty with 5 years. The result of these uncertainties is that the usage and the reliability of the product cannot be predicted. This is a high risk for the supplier that will be translated in high warranty costs. In the perfect SOLL situation however most of the variables will become fixed, known or manageable. Resulting from the GAP analysis several actions that influence the warranty costs for the supplier are summarized; develop a new warranty policy, conduct a research into the product reliability and develop a method to control the product usage to reduce the risk of intensive or improper use. To implement forward cooperation this is what needs to be done; develop a structure where the variables ‘product reliability’ and ‘product usage’ determine the warranty costs and where the ‘warranty policy’ is the last variable to define the definitive ‘warranty costs’. Thereby the ‘warranty policy’ should also influence the ‘product usage’. With the knowledge of the literature and the case study the following changes are made to the original model of D.N.P. Murthy and W.R. Blischke (2006) to create the eventual framework; (i) the supplier is now considered an integrator; (ii) cooperation with the sub-suppliers (backwards cooperation) is included; (iii) the warranty policies control the warranty costs by in- or excluding parts, non- or renewing warranty and a possible decrease in the coverage of warranty costs for the integrator over time; (iv) the product usage is influenced by the warranty policy so that the usage risks are (more) controlled by the integrator. In the framework in figure 1 the warranty costs are completely dependent of the product performance, which in turn is determined by the product reliability and the product usage. To make a reliable estimation of the covered warranty costs it is of great importance that both the product reliability and the product usage are known. Furthermore the framework shows that the supplier of dredging equipment can rather be seen as an ‘integrator’ that relies on the products of sub-suppliers, therefore the supplier of dredging equipment needs to apply backward cooperation to achieve forward integration. To be able to influence the warranty costs the supplier of dredging equipment needs to offer several warranty policies that suit the product usage profile of the client. By providing different options the supplier is able to reduce the financial risks and the expected warranty costs. Before the supplier of dredging equipment is able to offer the extended warranty, it is highly recommended to take these actions that will form the basis for the extended warranty service; (1) start logging data on product usage and product reliability; (2) cooperate and negotiate with the sub-suppliers; (3) investigate the failure mode of all parts and systems; (4) determine the warranty cost (modelling); (5) define the warranty policies and apply these to determine the covered warranty costs. Only when these actions are undertaken extended warranty can be successfully implemented by the supplier of dredging equipment.Construction Management and EngineeringStructural EngineeringCivil Engineering and Geoscience
Quantum Nature of the Proton in Water-Hydroxyl Overlayers on Metal Surfaces
No full textUsing ab initio path-integral molecular dynamics, we show that water-hydroxyl overlayers on transition metal surfaces exhibit surprisingly pronounced quantum nuclear effects. The metal substrates serve to reduce the classical proton transfer barriers within the overlayers and, in analogy to ice under high pressure, to shorten the corresponding intermolecular hydrogen bonds. Depending on the substrate and the intermolecular separations it imposes, the traditional distinction between covalent and hydrogen bonds is lost partially [e.g., on Pt(111) and Ru(0001)] or almost entirely [e.g., on Ni(111)]. We suggest that these systems provide an excellent platform on which to systematically explore the magnitude of quantum nuclear effects in hydrogen bonds
Electron and vibrational spectroscopies using DFT, plane waves and pseudopotentials : CASTEP implementation
No full textDensity functional theory can be used to interpret and predict spectroscopic properties of solid-state materials. The relevant computational solutions are usually available in disparate DFT codes, so that it is difficult to use a consistent approach for analyzing various spectroscopic features of a given material. We review the latest developments that are aimed to provide a collection of analytical tools within one DFT package, CASTEP. The applications covered include core-level EELS, solid-state NMR, optical properties, IR and Raman spectroscopy. We present also results of the EELS analysis of NbO and Nb 2O 5 that show the first published example of CASTEP spectra from d-states. Raman activities calculated for a test set of small molecules and the convergence requirements for such calculations are discussed. © 2010 Elsevier B.V
Synthesis of binary transition metal nitrides, carbides and borides from the elements in the laser-heated diamond anvil cell and their structure-property relations
No full textTransition metal nitrides, carbides and borides have a high potential for industrial applications as they not only have a high melting point but are generally harder and less compressible than the pure metals. Here we summarize recent advances in the synthesis of binary transition metal nitrides, carbides and borides focusing on the reaction of the elements at extreme conditions generated within the laser-heated diamond anvil cell. The current knowledge of their structures and high-pressure properties like high-(p; T) stability, compressibility and hardness is described as obtained from experiments
Reproducibility in density functional theory calculations of solids
No full textThe widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements
