483 research outputs found
Understanding the Initial Stages of Reversible Mg Deposition and Stripping in Inorganic Non-Aqueous Electrolytes
Multi-valent (MV) battery architectures based on pairing a Mg metal anode
with a high-voltage ( 3 V) intercalation cathode offer a realistic design
pathway toward significantly surpassing the energy storage performance of
traditional Li-ion based batteries, but there are currently only few
electrolyte systems that support reversible Mg deposition. Using both static
first-principles calculations and molecular dynamics, we perform
a comprehensive adsorption study of several salt and solvent species at the
interface of Mg metal with an electrolyte of Mg and Cl dissolved in
liquid tetrahydrofuran (THF). Our findings not only provide a picture of the
stable species at the interface, but also explain how this system can support
reversible Mg deposition and as such we provide insights in how to design other
electrolytes for Mg plating and stripping. The active depositing species are
identified to be (MgCl) monomers coordinated by THF, which exhibit
preferential adsorption on Mg compared to possible passivating species (such as
THF solvent or neutral MgCl complexes). Upon deposition, the energy to
desolvate these adsorbed complexes and facilitate charge-transfer is shown to
be small ( 61 46.2 kJ mol to remove 3 THF from the strongest
adsorbing complex), and the stable orientations of the adsorbed but desolvated
(MgCl) complexes appear favorable for charge-transfer. Finally,
observations of Mg-Cl dissociation at the Mg surface at very low THF
coordinations (0 and 1) suggest that deleterious Cl incorporation in the anode
may occur upon plating. In the stripping process, this is beneficial by further
facilitating the Mg removal reaction
Coarse-graining protein energetics in sequence variables
We show that cluster expansions (CE), previously used to model solid-state
materials with binary or ternary configurational disorder, can be extended to
the protein design problem. We present a generalized CE framework, in which
properties such as energy can be unambiguously expanded in the amino-acid
sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g.,
side-chain conformations) and thereby simplifies the problem of designing
proteins, or predicting the compatibility of a sequence with a given structure,
by many orders of magnitude. The CE is physically transparent, and can be
evaluated through linear regression on the energies of training sequences. We
show, as example, that good prediction accuracy is obtained with up to pairwise
interactions for a coiled-coil backbone, and that triplet interactions are
important in the energetics of a more globular zinc-finger backbone.Comment: 10 pages, 3 figure
Predicting Crystal Structures with Data Mining of Quantum Calculations
Predicting and characterizing the crystal structure of materials is a key
problem in materials research and development. It is typically addressed with
highly accurate quantum mechanical computations on a small set of candidate
structures, or with empirical rules that have been extracted from a large
amount of experimental information, but have limited predictive power. In this
letter, we transfer the concept of heuristic rule extraction to a large library
of ab-initio calculated information, and demonstrate that this can be developed
into a tool for crystal structure prediction.Comment: 4 pages, 3 pic
Non-ohmicity and energy relaxation in diffusive 2D metals
We analyze current-voltage characteristics taken on Au-doped indium-oxide
films. By fitting a scaling function to the data, we extract the
electron-phonon scattering rate as function of temperature, which yields a
quadratic dependence of the electron-phonon scattering rate on temperature from
1K down to 0.28K. The origin of this enhanced electron-phonon scattering rate
is ascribed to the mechanism proposed by Sergeev and Mitin.Comment: 7 pages, 6 figure
Dynamic of a non homogeneously coarse grained system
To study materials phenomena simultaneously at various length scales,
descriptions in which matter can be coarse grained to arbitrary levels, are
necessary. Attempts to do this in the static regime (i.e. zero temperature)
have already been developed. In this letter, we present an approach that leads
to a dynamics for such coarse-grained models. This allows us to obtain
temperature-dependent and transport properties. Renormalization group theory is
used to create new local potentials model between nodes, within the
approximation of local thermodynamical equilibrium. Assuming that these
potentials give an averaged description of node dynamics, we calculate thermal
and mechanical properties. If this method can be sufficiently generalized it
may form the basis of a Molecular Dynamics method with time and spatial
coarse-graining.Comment: 4 pages, 4 figure
Ab initio investigation of ammonia-borane complexes for hydrogen storage
The structural, electronic, and thermodynamic properties of ammonia-borane complexes with varying amounts of hydrogen have been characterized by first principles calculations within density functional theory. The calculated structural parameters and thermodynamic functions ͑free energy, enthalpy and entropy͒ were found to be in good agreement with experimental and quantum chemistry data for the crystals, dimers, and molecules. The authors find that zero-point energies change several H 2 release reactions from endothermic to exothermic. Both the ammonia-borane polymeric and borazine-cyclotriborazane cycles show a strong exothermic decomposition character ͑approximately −10 kcal/ mol͒, implying that rehydrogenation may be difficult to moderate H 2 pressures. Hydrogen bonding in these systems has been characterized and they find the N-H bond to be more covalent than the more ionic B-H bond
Ternary Nitride Semiconductors in the Rocksalt Crystal Structure
Inorganic nitrides with wurtzite crystal structures are well-known
semiconductors used in optoelectronic devices. In contrast, rocksalt-based
nitrides are known for their metallic and refractory properties. Breaking this
dichotomy, here we report on ternary nitride semiconductors with rocksalt
crystal structures, remarkable optoelectronic properties, and the general
chemical formula MgTMN (TM=Ti, Zr, Hf, Nb). These compounds form
over a broad metal composition range and our experiments show that Mg-rich
compositions are nondegenerate semiconductors with visible-range optical
absorption onsets (1.8-2.1 eV). Lattice parameters are compatible with growth
on a variety of substrates, and epitaxially grown MgZrN exhibits
remarkable electron mobilities approaching 100 cmVs. Ab
initio calculations reveal that these compounds have disorder-tunable optical
properties, large dielectric constants and low carrier effective masses that
are insensitive to disorder. Overall, these experimental and theoretical
results highlight MgTMN rocksalts as a new class of
semiconductor materials with promising properties for optoelectronic
applications
First-principles prediction of redox potentials in transition-metal compounds with LDA+U
First-principles calculations within the Local Density Approximation (LDA) or
Generalized Gradient Approximation (GGA), though very successful, are known to
underestimate redox potentials, such as those at which lithium intercalates in
transition metal compounds. We argue that this inaccuracy is related to the
lack of cancellation of electron self-interaction errors in LDA/GGA and can be
improved by using the DFT+ method with a self-consistent evaluation of the
parameter. We show that, using this approach, the experimental lithium
intercalation voltages of a number of transition metal compounds, including the
olivine LiMPO (M=Mn, Fe Co, Ni), layered LiMO (Co,
Ni) and spinel-like LiMO (M=Mn, Co), can be reproduced
accurately.Comment: 19 pages, 6 figures, Phys. Rev. B 70, 235121 (2004
S=1/2 chains and spin-Peierls transition in TiOCl
We study TiOCl as an example of an S=1/2 layered Mott insulator. From our
analysis of new susceptibility data, combined with LDA and LDA+U band structure
calculations, we conclude that orbital ordering produces quasi-one-dimensional
spin chains and that TiOCl is a new example of Heisenberg-chains which undergo
a spin-Peierls transition. The energy scale is an order of magnitude larger
than that of previously known examples. The effects of non-magnetic Sc
impurities are explained using a model of broken finite chains.Comment: 5 pages, 5 figures (color); details on crystal growth added; to be
published in Phys. Rev.
Ultra-Fast Evaluation of Protein Energies Directly from Sequence
The structure, function, stability, and many other properties of a protein in a fixed environment are fully specified by its sequence, but in a manner that is difficult to discern. We present a general approach for rapidly mapping sequences directly to their energies on a pre-specified rigid backbone, an important sub-problem in computational protein design and in some methods for protein structure prediction. The cluster expansion (CE) method that we employ can, in principle, be extended to model any computable or measurable protein property directly as a function of sequence. Here we show how CE can be applied to the problem of computational protein design, and use it to derive excellent approximations of physical potentials. The approach provides several attractive advantages. First, following a one-time derivation of a CE expansion, the amount of time necessary to evaluate the energy of a sequence adopting a specified backbone conformation is reduced by a factor of 10(7) compared to standard full-atom methods for the same task. Second, the agreement between two full-atom methods that we tested and their CE sequence-based expressions is very high (root mean square deviation 1.1–4.7 kcal/mol, R(2) = 0.7–1.0). Third, the functional form of the CE energy expression is such that individual terms of the expansion have clear physical interpretations. We derived expressions for the energies of three classic protein design targets—a coiled coil, a zinc finger, and a WW domain—as functions of sequence, and examined the most significant terms. Single-residue and residue-pair interactions are sufficient to accurately capture the energetics of the dimeric coiled coil, whereas higher-order contributions are important for the two more globular folds. For the task of designing novel zinc-finger sequences, a CE-derived energy function provides significantly better solutions than a standard design protocol, in comparable computation time. Given these advantages, CE is likely to find many uses in computational structural modeling
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