| 1. |
- Bergman, Anders, et al.
(författare)
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Magnetic properties of Fe/Co(001) superlattices from first-principles theory
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:17
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of Fe/Co(001) superlattices have been studied using fully-relativistic first-principles theories. The average magnetic moment shows a behavior similar to bulk Fe-Co alloys, i.e., an enhanced magnetic moment for low Co concentrations, as described by the Slater-Pauling curve. The maximum of the magnetization curve, however, is lowered and shifted towards the Fe-rich compositions. The increased average magnetic moment for the Fe-rich superlattices, compared to bulk Fe, is due to an enhancement of the Fe spin moment close to the Fe-Co interface. The orbital moments were found to be of the same size as in bulk. The effect of interface roughness on the magnetic properties was investigated, and it was found that-despite local fluctuations due to the varying coordination-the average magnetic moment is only slightly affected. From a mapping of first-principles interactions onto the screened generalized perturbation method, we calculate the temperatures for when Fe/Co superlattices break up into an alloy configuration. Furthermore, the tetragonal distortion of the superlattice structure was found to only have a minor effect on the magnetic moments. Also, the calculated easy axis of magnetization is in the film plane for all compositions studied. It lies along the [100] direction for Fe-rich superlattices and along the [110] direction for Co-rich compositions. The transition of the easy axis occurs around a Co concentration of 50%.
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| 2. |
- Eriksson, Anders, 1975, et al.
(författare)
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Bottom-up derivation of an effective thermostat for united atoms simulations of water
- 2009
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Ingår i: JOURNAL OF CHEMICAL PHYSICS. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:16
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Tidskriftsartikel (refereegranskat)abstract
- In this article we derive the effective pairwise interactions in a Langevin-type united atoms model of water. The interactions are determined from the trajectories of a detailed molecular dynamics simulation of simple point charge water. A standard method is used for estimating the conservative interaction, whereas a new "bottom-up" method is used to determine the effective dissipative and stochastic interactions. We demonstrate that when compared to the standard united atoms model, the transport properties of the coarse-grained model is significantly improved by the introduction of the derived dissipative and stochastic interactions. The results are compared to a previous study, where a "top-down" approach was used to obtain transport properties consistent with those of the simple point charge water model. © 2009 American Institute of Physics.
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| 3. |
- Eriksson, Anders, 1975, et al.
(författare)
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Effective thermostat induced by coarse graining of simple point charge water
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how the transport properties of a united atom fluid with a dissipative particle dynamics thermostat depend on the functional form and magnitude of both the conservative and the stochastic interactions. We demonstrate how the thermostat strongly affects the hydrodynamics, especially diffusion, viscosity, and local escape times. As model system we use simple point charge (SPC) water, from which projected trajectories are used to determine the effective interactions in the united atom model. The simulation results support our argument that the thermostat should be viewed as an integral part of the coarse-grained dynamics rather than a tool for approaching thermal equilibrium. As our main result we show that the united atom model with the adjusted effective interactions approximately reproduces the diffusion constant and the viscosity of the underlying detailed SPC water model.
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| 4. |
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| 5. |
- Eriksson, Anders, 1975, et al.
(författare)
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On the microscopic foundation of dissipative particle dynamics
- 2009
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 86:4
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Tidskriftsartikel (refereegranskat)abstract
- Mesoscopic-particle-based fluid models, such as dissipative particle dynamics, are usually assumed to be coarse-grained representations of an underlying microscopic fluid. A fundamental question is whether there exists a map from microscopic particles in these systems to the corresponding coarse-grained particles, such that the coarse-grained system has the same bulk and transport properties as the underlying system. In this letter, we investigate the coarse-graining of microscopic fluids using a Voronoi-type projection that has been suggested in several studies. The simulations show that the projection fails in defining coarse-grained particles that have a physically meaningful connection to the microscopic fluid. In particular, the Voronoi projection produces identical coarse-grained equilibrium properties when applied to systems with different microscopic interactions and different bulk properties. © EPLA, 2009
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| 6. |
- Eriksson, Anders, 1975, et al.
(författare)
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Using force covariance to derive effective stochastic interactions in dissipative particle dynamics.
- 2008
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 77:1
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Tidskriftsartikel (refereegranskat)abstract
- There exist methods for determining effective conservative interactions in coarse-grained particle-based mesoscopic simulations. The resulting models can be used to capture thermal equilibrium behavior, but the model system we study does not correctly represent transport properties. We suggest the use of force covariance to determine the full functional form of dissipative and stochastic interactions. We show that a combination of the RDF and a force covariance function can be used to determine all interactions in dissipative particle dynamics (DPD). Furthermore, we use the method to test whether the effective interactions in DPD can be adjusted to produce a force covariance consistent with the projection of a microscopic Lennard-Jones simulation. The results indicate that the DPD ansatz may not be consistent with the underlying microscopic dynamics. We discuss how this result relates to theoretical studies reported in the literature.
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| 7. |
- Heintz, Sofia, 1972-
(författare)
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Muscular forces from static optimization
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At every joint there is a redundant set of muscle activated during movement or loading of the system. Optimization techniques are needed to evaluate individual forces in every muscle. The objective in this thesis was to use static optimization techniques to calculate individual muscle forces in the human extremities. A cost function based on a performance criterion of the involved muscular forces was set to be minimized together with constraints on the muscle forces, restraining negative and excessive values. Load-sharing, load capacity and optimal forces of a system can be evaluated, based on a description of the muscle architectural properties, such as moment arm, physiological cross-sectional area, and peak isometric force. The upper and lower extremities were modelled in two separate studies. The upper extremity was modelled as a two link-segment with fixed configurations. Load-sharing properties in a simplified model were analyzed. In a more complex model of the elbow and shoulder joint system of muscular forces, the overall total loading capacity was evaluated. A lower limb model was then used and optimal forces during gait were evaluated. Gait analysis was performed with simultaneous electromyography (EMG). Gait kinematics and kinetics were used in the static optimization to evaluate of optimal individual muscle forces. EMG recordings measure muscle activation. The raw EMG data was processed and a linear envelope of the signal was used to view the activation profile. A method described as the EMG-to-force method which scales and transforms subject specific EMG data is used to compare the evaluated optimal forces. Reasonably good correlation between calculated muscle forces from static optimization and EMG profiles was shown. Also, the possibility to view load-sharing properties of a musculoskeletal system demonstrate a promising complement to traditional motion analysis techniques. However, validation of the accurate muscular forces are needed but not possible. Future work is focused on adding more accurate settings in the muscle architectural properties such as moment arms and physiological cross-sectional areas. Further perspectives with this mathematic modelling technique include analyzing pathological movement, such as cerebral palsy and rheumatoid arthritis where muscular weakness, pain and joint deformities are common. In these, better understanding of muscular action and function are needed for better treatment.
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| 8. |
- Hörnberg, Andreas, 1972-, et al.
(författare)
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The effect of iodide and chloride on transthyretin structure and stability
- 2005
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 44:26, s. 9290-9299
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Transthyretin amyloid formation occurs through a process of tetramer destabilization and partial unfolding. Small molecules, including the natural ligand thyroxine, stabilize the tetrameric form of the protein, and serve as inhibitors of amyloid formation. Crucial for TTR's ligand-binding properties are its three halogen-binding sites situated at the hormone-binding channel. In this study, we have performed a structural characterization of the binding of two halides, iodide and chloride, to TTR. Chlorides are known to shield charge repulsions at the tetrameric interface of TTR, which improve tetramer stability of the protein. Our study shows that iodides, like chlorides, provide tetramer stabilization in a concentration-dependent manner and at concentrations approximately 15-fold below that of chlorides. To elucidate binding sites of the halides, we took advantage of the anomalous scattering of iodide and used the single-wavelength anomalous dispersion (SAD) method to solve the iodide-bound TTR structure at 1.8 A resolution. The structure of chloride-bound TTR was determined at 1.9 A resolution using difference Fourier techniques. The refined structures showed iodides and chlorides bound at two of the three halogen-binding sites located at the hydrophobic channel. These sites therefore also function as halide-binding sites.
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