321 research outputs found
Escaping free-energy minima
We introduce a novel and powerful method for exploring the properties of the
multidimensional free energy surfaces of complex many-body systems by means of
a coarse-grained non-Markovian dynamics in the space defined by a few
collective coordinates.A characteristic feature of this dynamics is the
presence of a history-dependent potential term that, in time, fills the minima
in the free energy surface, allowing the efficient exploration and accurate
determination of the free energy surface as a function of the collective
coordinates. We demonstrate the usefulness of this approach in the case of the
dissociation of a NaCl molecule in water and in the study of the conformational
changes of a dialanine in solution.Comment: 3 figure
Ab initio simulations of the Ag(111)/Al2O3 interface at intermediate oxygen partial pressures
The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental dat
Role of interface coupling inhomogeneity in domain evolution in exchange bias
Models of exchange-bias in thin films have been able to describe various
aspects of this technologically relevant effect. Through appropriate choices of
free parameters the modelled hysteresis loops adequately match experiment, and
typical domain structures can be simulated. However, the use of these
parameters, notably the coupling strength between the systems' ferromagnetic
(F) and antiferromagnetic (AF) layers, obscures conclusions about their
influence on the magnetization reversal processes. Here we develop a 2D
phase-field model of the magnetization process in exchange-biased CoO/(Co/Pt)xn
that incorporates the 10 nm-resolved measured local biasing characteristics of
the antiferromagnet. Just three interrelated parameters set to measured
physical quantities of the ferromagnet and the measured density of
uncompensated spins thus suffice to match the experiment in microscopic and
macroscopic detail. We use the model to study changes in bias and coercivity
caused by different distributions of pinned uncompensated spins of the
antiferromagnet, in application-relevant situations where domain wall motion
dominates the ferromagnetic reversal. We show the excess coercivity can arise
solely from inhomogeneity in the density of biasing- and anti-biasing pinned
uncompensated spins in the antiferromagnet. Counter to conventional wisdom,
irreversible processes in the latter are not essential
A simple approach for describing metal-supported cyclohexaphenylene dehydrogenation: Hybrid classical/DFT metadynamics simulations
The mechanisms for the dehydrogenation reaction of cyclohexaphenylene at a copper surface are investigated with the help of density functional theory and metadynamics. Our results represent a showcase for an approach that describes the surface using many-body classical potentials and molecule-surface interactions with a van der Waals model. Starting from the experimental observation that dispersion-assisted mechanisms are at least as important as catalytic processes for the description of the reaction, we fully describe the former, we identify intermediate states and estimate the free energy barriers that characterize the reactio
Self-assembly and two-dimensional spontaneous resolution of cyano-functionalized [7]helicenes on Cu111
Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CNâ‹…â‹…â‹…HC(Ar) hydrogen bonding and CNâ‹…â‹…â‹…CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules
An optimizing C front-end for hardware synthesis
Modern embedded systems must execute a variety of high performance real-time tasks, such as audio and image compression and decompression, channel coding and encoding, etc. High hardware design and mask production costs dictate the need to re-use an architectural platform for as many applications as possible. Reconfigurable platforms can be very effective in these cases, because they allow one to re-use the architecture across a variety of applications. The efficient use of a reconfigurable platform requires a methodology and tools supporting it in order to extensively explore the hardware/software design space, without requiring developers to have a deep knowledge of the underlying architecture, since they often have a software background and only limited hardware design skills. This paper describes a tool that fits into a complete design flow for a reconfigurable processor and that allows one to efficiently transform a high level specification into a lower level one, more suitable for synthesis on the reconfigurable array. The effectiveness of the methodology is proved by a complete implementation of a turbo-decoder
Two-Stage Rotational Disordering of a Molecular Crystal Surface: C\u3csub\u3e60\u3c/sub\u3e
We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C60 fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2Ă—2) state, and a high-temperature (1Ă—1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces
- …