4,332 research outputs found
How the Choice of Force-Field Affects the Stability and Self-Assembly Process of Supramolecular CTA Fibers
[Image: see text] In recent years, computational methods have become an essential element of studies focusing on the self-assembly process. Although they provide unique insights, they face challenges, from which two are the most often mentioned in the literature: the temporal and spatial scale of the self-assembly. A less often mentioned issue, but not less important, is the choice of the force-field. The repetitive nature of the supramolecular structure results in many similar interactions. Consequently, even a small deviation in these interactions can lead to significant energy differences in the whole structure. However, studies comparing different force-fields for self-assembling systems are scarce. In this article, we compare molecular dynamics simulations for trifold hydrogen-bonded fibers performed with different force-fields, namely GROMOS, CHARMM General Force Field (CGenFF), CHARMM Drude, General Amber Force-Field (GAFF), Martini, and polarized Martini. Briefly, we tested the force-fields by simulating: (i) spontaneous self-assembly (none form a fiber within 500 ns), (ii) stability of the fiber (observed for CHARMM Drude, GAFF, MartiniP), (iii) dimerization (observed for GROMOS, GAFF, and MartiniP), and (iv) oligomerization (observed for CHARMM Drude and MartiniP). This system shows that knowledge of the force-field behavior regarding interactions in oligomer and larger self-assembled structures is crucial for designing efficient simulation protocols for self-assembling systems
Large tunable image-charge effects in single-molecule junctions
The characteristics of molecular electronic devices are critically determined
by metal-organic interfaces, which influence the arrangement of the orbital
levels that participate in charge transport. Studies on self-assembled
monolayers (SAMs) show (molecule-dependent) level shifts as well as
transport-gap renormalization, suggesting that polarization effects in the
metal substrate play a key role in the level alignment with respect to the
metal's Fermi energy. Here, we provide direct evidence for an electrode-induced
gap renormalization in single-molecule junctions. We study charge transport in
single porphyrin-type molecules using electrically gateable break junctions. In
this set-up, the position of the occupied and unoccupied levels can be followed
in situ and with simultaneous mechanical control. When increasing the electrode
separation, we observe a substantial increase in the transport gap with level
shifts as high as several hundreds of meV for displacements of a few \aa
ngstroms. Analysis of this large and tunable gap renormalization with
image-charge calculations based on atomic charges obtained from density
functional theory confirms and clarifies the dominant role of image-charge
effects in single-molecule junctions
Hole spin polarization in GaAlAs:Mn structures
A self-consistent calculation of the electronic properties of GaAlAs:Mn
magnetic semiconductor quantum well structures is performed including the
Hartree term and the sp-d exchange interaction with the Mn magnetic moments.
The spin polarization density is obtained for several structure configurations.
Available experimental results are compared with theory.Comment: 4 page
Theory of Diluted Magnetic Semiconductor Ferromagnetism
We present a theory of carrier-induced ferromagnetism in diluted magnetic
semiconductors (III_{1-x} Mn_x V) which allows for arbitrary itinerant-carrier
spin polarization and dynamic correlations. Both ingredients are essential in
identifying the system's elementary excitations and describing their
properties. We find a branch of collective modes, in addition to the spin waves
and Stoner continuum which occur in metallic ferromagnets, and predict that the
low-temperature spin stiffness is independent of the strength of the exchange
coupling between magnetic ions and itinerant carriers. We discuss the
temperature dependence of the magnetization and the heat capacity
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