1,868 research outputs found
Self-learning Kinetic Monte-Carlo method: application to Cu(111)
We present a novel way of performing kinetic Monte Carlo simulations which
does not require an {\it a priori} list of diffusion processes and their
associated energetics and reaction rates.
Rather, at any time during the simulation, energetics for all possible
(single or multi-atom) processes, within a specific interaction range, are
either computed accurately using a saddle point search procedure, or retrieved
from a database in which previously encountered processes are stored. This
self-learning procedure enhances the speed of the simulations along with a
substantial gain in reliability because of the inclusion of many-particle
processes.
Accompanying results from the application of the method to the case of
two-dimensional Cu adatom-cluster diffusion and coalescence on Cu(111) with
detailed statistics of involved atomistic processes and contributing diffusion
coefficients attest to the suitability of the method for the purpose.Comment: 18 pages, 9 figure
Hydrogen Absorption Properties of Metal-Ethylene Complexes
Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a
single ethylene molecule can form stable complexes with light transition metals
(TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and
14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to
include a large number of other metals and different isomeric structures. We
obtained interesting results for light metals such as Li. The ethylene molecule
is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which
then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2
and absorption capacity of 16 wt %, a record high value reported so far. The
stability of the proposed metal-ethylene complexes was tested by extensive
calculations such as normal-mode analysis, finite temperature first-principles
molecular dynamics (MD) simulations, and reaction path calculations. The phonon
and MD simulations indicate that the proposed structures are stable up to 500
K. The reaction path calculations indicate about 1 eV activation barrier for
the TM2-ethylene complex to transform into a possible lower energy
configuration where the ethylene molecule is dissociated. Importantly, no
matter which isometric configuration the TM2-ethylene complex possesses, the TM
atoms are able to bind multiple hydrogen molecules with suitable binding energy
for room temperature storage. These results suggest that co-deposition of
ethylene with a suitable precursor of TM or Li into nanopores of light-weight
host materials may be a very promising route to discovering new materials with
high-capacity hydrogen absorption properties
Multiple-mouse Neuroanatomical Magnetic Resonance Imaging
The field of mouse phenotyping with magnetic resonance imaging (MRI) is rapidly growing, motivated by the need for improved tools for characterizing and evaluating mouse models of human disease. MRI is an excellent modality for investigating genetically altered animals. It is capable of whole brain coverage, can be used in vivo, and provides multiple contrast mechanisms for investigating different aspects of neuranatomy and physiology. The advent of high-field scanners along with the ability to scan multiple mice simultaneously allows for rapid phenotyping of novel mutations
Conformational Dependence of a Protein Kinase Phosphate Transfer Reaction
Atomic motions and energetics for a phosphate transfer reaction catalyzed by
the cAMP-dependent protein kinase (PKA) are calculated by plane-wave density
functional theory, starting from structures of proteins crystallized in both
the reactant conformation (RC) and the transition-state conformation (TC). In
the TC, we calculate that the reactants and products are nearly isoenergetic
with a 0.2 eV barrier; while phosphate transfer is unfavorable by over 1.2 eV
in the RC, with an even higher barrier. With the protein in the TC, the motions
involved in reaction are small, with only P and the catalytic proton
moving more than 0.5 \AA. Examination of the structures reveals that in the RC
the active site cleft is not completely closed and there is insufficient space
for the phosphorylated serine residue in the product state. Together, these
observations imply that the phosphate transfer reaction occurs rapidly and
reversibly in a particular conformation of the protein, and that the reaction
can be gated by changes of a few tenths of an \AA in the catalytic site.Comment: revtex4, 7 pages, 4 figures, to be submitted to Scienc
Investigating Rare Events by Transition Interface Sampling
We briefly review simulation schemes for the investigation of rare
transitions and we resume the recently introduced Transition Interface
Sampling, a method in which the computation of rate constants is recast into
the computation of fluxes through interfaces dividing the reactant and product
state.Comment: 12 pages, 1 figure, contributed paper to the proceedings of NEXT
2003, Second Sardinian International Conference on News and Expectations in
Thermostatistics, 21-28 Sep 2003, Cagliari (Italy
A new triclinic modification of the pyrochlore-type KOs2O6 superconductor
A new modification of KOs2O6, the representative of a new structural type
(Pearson symbol aP18, a=5.5668(1)A, b=6.4519(2)A, c=7.2356(2)A, space group
P-1, no.2) was synthesized employing high pressure technique. Its structure was
determined by single-crystal X-ray diffraction. The structure can be described
as two OsO6 octahedral chains relating to each other through inversion and
forming big voids with K atoms inside. Quantum chemical calculations were
performed on the novel compound and structurally related cubic compound.
High-pressure X-ray study showed that cubic KOs2O6 phase was stable up to
32.5(2) GPa at room temperature.Comment: 23 pages, 9 figures,6 tables. Accepted for J. Solid State Che
DFT Study of Planar Boron Sheets: A New Template for Hydrogen Storage
We study the hydrogen storage properties of planar boron sheets and compare
them to those of graphene. The binding of molecular hydrogen to the boron sheet
(0.05 eV) is stronger than that to graphene. We find that dispersion of alkali
metal (AM = Li, Na, and K) atoms onto the boron sheet markedly increases
hydrogen binding energies and storage capacities. The unique structure of the
boron sheet presents a template for creating a stable lattice of strongly
bonded metal atoms with a large nearest neighbor distance. In contrast, AM
atoms dispersed on graphene tend to cluster to form a bulk metal. In particular
the boron-Li system is found to be a good candidate for hydrogen storage
purposes. In the fully loaded case this compound can contain up to 10.7 wt. %
molecular hydrogen with an average binding energy of 0.15 eV/H2.Comment: 19 pages, 7 figures, and 3 table
Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W
Screw dislocations in bcc metals display non-planar cores at zero temperature
which result in high lattice friction and thermally activated strain rate
behavior. In bcc W, electronic structure molecular statics calculations reveal
a compact, non-degenerate core with an associated Peierls stress between 1.7
and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations
can only be gained by using more efficient atomistic simulations based on
semiempirical interatomic potentials. In this paper we assess the suitability
of five different potentials in terms of static properties relevant to screw
dislocations in pure W. As well, we perform molecular dynamics simulations of
stress-assisted glide using all five potentials to study the dynamic behavior
of screw dislocations under shear stress. Dislocations are seen to display
thermally-activated motion in most of the applied stress range, with a gradual
transition to a viscous damping regime at high stresses. We find that one
potential predicts a core transformation from compact to dissociated at finite
temperature that affects the energetics of kink-pair production and impacts the
mechanism of motion. We conclude that a modified embedded-atom potential
achieves the best compromise in terms of static and dynamic screw dislocation
properties, although at an expense of about ten-fold compared to central
potentials
The Energy Landscape, Folding Pathways and the Kinetics of a Knotted Protein
The folding pathway and rate coefficients of the folding of a knotted protein
are calculated for a potential energy function with minimal energetic
frustration. A kinetic transition network is constructed using the discrete
path sampling approach, and the resulting potential energy surface is
visualized by constructing disconnectivity graphs. Owing to topological
constraints, the low-lying portion of the landscape consists of three distinct
regions, corresponding to the native knotted state and to configurations where
either the N- or C-terminus is not yet folded into the knot. The fastest
folding pathways from denatured states exhibit early formation of the
N-terminus portion of the knot and a rate-determining step where the C-terminus
is incorporated. The low-lying minima with the N-terminus knotted and the
C-terminus free therefore constitute an off-pathway intermediate for this
model. The insertion of both the N- and C-termini into the knot occur late in
the folding process, creating large energy barriers that are the rate limiting
steps in the folding process. When compared to other protein folding proteins
of a similar length, this system folds over six orders of magnitude more
slowly.Comment: 19 page
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