Modelling radiation effects in solids with two-temperature molecular dynamics

The ability to predict the structural modifications of materials resulting from a broad range of irradiation scenarios would have a positive impact on many fields of science and technology. Established techniques for modelling large atomic systems, such as classical molecular dynamics, are limited by the neglect of the electronic degrees of freedom which restricts their application to irradiation events that primarily interact with atomic nuclei. Ab initio methods, on the other hand, include electronic degrees of freedom, but the requisite computational costs restrict their application to relatively small systems. Recent methodological developments aimed at overcoming some of these limitations are based on methods that couple atomistic models to a continuum model for the electronic energy, where energy is exchanged between the nuclei and electrons via electronic stopping and electron-phonon coupling mechanisms. Such two-temperature molecular dynamics models, as they are known, make it practicable to simulate the effects of electronic excitations on systems with millions, or even hundreds of millions, of atoms. They have been used to study laser irradiation of metallic films, swift heavy ion irradiation of metals and semiconductors, and moderately high ion irradiation of metals. In this review we describe the two-temperature molecular dynamics methodology and the various practical considerations required for its implementation. We provide example applications of the model to multiple irradiation scenarios that accommodate electronic excitations. We also describe the challenges of including the effects of the modification of the interatomic interactions, due to the excitation of electrons, in the simulations and how these challenges can be overcome

Event Management Skills.

So, how do we counter the criticism of the Taxpayer’s Alliance that events related courses could be described as ‘non-courses’? Sometimes it feels like we are caught in the middle of what industry wants and what gives our courses academic credibility i.e. the ongoing vocational (professional) vs academic argument. Does it have to be this way? The aim of this article is to stimulate debate amongst the events community and to highlight some of the current developments in both Higher Education and ‘industry’ (i.e. Sector Skills) that might be relevant to us. To add to the discussion we bring in some initial findings from research that we are undertaking with event organisations as to which skills they think are the most important for managers of events

Accelerating Solvent Dynamics with Replica Exchange for Improved Free Energy Sampling

Molecular reactions in solution typically involve solvent exchange; for example, a surface must partly desolvate for a molecule to adsorb onto it. When these reactions are simulated, slow solvent dynamics can limit the sampling of configurations and reduce the accuracy of free energy estimates. Here, we combine Hamiltonian replica exchange (HREX) with well-tempered metadynamics (WTMD) to accelerate the sampling of solvent configurations orthogonal to the collective variable space. We compute the formation free energy of a carbonate vacancy in the calcite–water interface and find that the combination of WTMD with HREX significantly improves the sampling relative to WTMD without HREX

Computational insight into the molecular mechanisms that control the growth of inorganic crystals

After billions of years of evolution, nature has developed mechanisms for controlling the growth and assembly of materials right down to the nanoscale, an achievement that materials scientists hope to mimic. However, the underlying processes are extremely complex and depend on subtle behaviour at the molecular scale. In contrast to experimental methods, computer simulations can achieve the molecular resolution needed to investigate these mechanisms, and can therefore offer unique insight. Indeed, this dissertation employs a variety of state-of-the-art computational methodologies to investigate the molecular processes by which calcite, the most abundant biomineral on earth, grows, in addition to the role played by surfactants in soft templating technologically important inorganic materials. Microsecond-long simulations are performed to reveal the behaviour of individual ions in the vicinity of calcite steps, providing new insight into the mechanisms responsible for kink nucleation. Rare event methodologies are then used to study the dissolution process of kink sites in calcite crystals. It is discovered that this particular mineralisation process is too complex to be tamed by computational methods, which has far-reaching consequences for the development of highly predictive models of mineralisation. A coarse-grained model for calcite precipitation is presented that displays the ability to connect molecular processes with both the kinetic and morphological characters of a crystal. However, the simulation is found to conflict with experimental observations regarding the dependence of step velocity on step length. The implication being that present models are unable to correctly describe step pinning, which is a major limitation. Lastly, the role of surfactants in templating crystal growth via two very different mechanisms is investigated. In the one case, polymorph and orientation selection by self-assembled monolayers; and in the other, oriented heterogeneous nucleation of mesoporous organosilicas

Calcite Kinks Grow via a Multistep Mechanism

The classical model of crystal growth assumes that kinks grow via a sequence of independent adsorption events where each solute transitions from the solution directly to the crystal lattice site. Here, we challenge this view by showing that some calcite kinks grow via a multistep mechanism where the solute adsorbs to an intermediate site and only transitions to the lattice site upon the adsorption of a second solute. We compute the free energy curves for Ca and CO3 ions adsorbing to a large selection of kink types, and we identify kinks terminated both by Ca ions and by CO3 ions that grow in this multistep way

Nonequilibrium capture of impurities that completely block kinks during crystal growth

Some impurities cannot integrate into isolated kinks because they completely block the growth of the kinks to which they adsorb. For this class of impurity, we derive an equation for the amount that incorporates into a crystal during growth of the elementary step by assuming that such an impurity incorporates if and only if it gets captured between a kink and an antikink. We show that the impurity concentration in the crystal increases monotonically with the impurity concentration in the mother phase, but that it can vary non-monotonically with both the supersaturation of the mother phase and the kink density of the step. In contrast to other capture mechanisms, we find that weakly adsorbed impurities incorporate to an extent that is independent of the supersaturation when the supersaturation is high. Irrespective of the growth conditions, the amount of impurity that can incorporate into a crystal is limited by an upper bound determined by the kink density

Diagnosis and Decision-Making in Telemedicine

This article provides an analysis of the skills that health professionals and patients employ in reaching diagnosis and decision-making in telemedicine consultations. As governmental priorities continue to emphasize patient involvement in the management of their disease, there is an increasing need to accurately capture the provider–patient interactions in clinical encounters. Drawing on conversation analysis of 10 video-mediated consultations in 3 National Health Service settings in England, this study examines the interaction between patients, General Practitioner (GPs), nurses, and consultants during diagnosis and decision-making, with the aim to identify the range of skills that participants use in the process and capture the interprofessional communication and patient involvement in the diagnosis and decision-making phases of telemedicine consultations. The analysis shows that teleconsultations enhance collaborative working among professionals and enable GPs and nurses to develop their skills and actively participate in diagnosis and decision-making by contributing primary care–specific knowledge to the consultation. However, interprofessional interaction may result in limited patient involvement in decisionmaking. The findings of this study can be used to inform training programs in telemedicine that focus on the development of effective skills for professionals and the provision of information to patients

The effect of surface topography on the micellisation of hexadecyltrimethylammonium chloride at the silicon-aqueous interface

Amphiphilic aggregation at solid-liquid interfaces can generate mesostructured micelles that can serve as soft templates. In this study we have simulated the self-assembly of hexadecyltrimethylammonium chloride (C16TAC) surfactants at the Si(1 0 0)- and Si(1 1 1)-aqueous interfaces. The surfactants are found to form semicylindrical micelles on Si(1 0 0) but hemispherical micelles on Si(1 1 1). This difference in micelle structure is shown to be a consequence of the starkly different surface topographies that result from the reconstruction of the two silicon surfaces, and reveals that micelle structure can be governed by epitaxial matching even with non-polar substrates