272 research outputs found
Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties
Graphene nanoribbons are the counterpart of carbon nanotubes in
graphene-based nanoelectronics. We investigate the electronic properties of
chemically modified ribbons by means of density functional theory. We observe
that chemical modifications of zigzag ribbons can break the spin degeneracy.
This promotes the onset of a semiconducting-metal transition, or of an
half-semiconducting state, with the two spin channels having a different
bandgap, or of a spin-polarized half-semiconducting state -where the spins in
the valence and conduction bands are oppositely polarized. Edge
functionalization of armchair ribbons gives electronic states a few eV away
from the Fermi level, and does not significantly affect their bandgap. N and B
produce different effects, depending on the position of the substitutional
site. In particular, edge substitutions at low density do not significantly
alter the bandgap, while bulk substitution promotes the onset of
semiconducting-metal transitions. Pyridine-like defects induce a
semiconducting-metal transition.Comment: 12 pages, 5 figure
Machine learning of microscopic structure-dynamics relationships in complex molecular systems
In many complex molecular systems, the macroscopic ensemble's properties are
controlled by microscopic dynamic events (or fluctuations) that are often
difficult to detect via pattern-recognition approaches. Discovering the
relationships between local structural environments and the dynamical events
originating from them would allow unveiling microscopic level
structure-dynamics relationships fundamental to understand the macroscopic
behavior of complex systems. Here we show that, by coupling advanced structural
(e.g., Smooth Overlap of Atomic Positions, SOAP) with local dynamical
descriptors (e.g., Local Environment and Neighbor Shuffling, LENS) in a unique
dataset, it is possible to improve both individual SOAP- and LENS-based
analyses, obtaining a more complete characterization of the system under study.
As representative examples, we use various molecular systems with diverse
internal structural dynamics. On the one hand, we demonstrate how the
combination of structural and dynamical descriptors facilitates decoupling
relevant dynamical fluctuations from noise, overcoming the intrinsic limits of
the individual analyses. Furthermore, machine learning approaches also allow
extracting from such combined structural/dynamical dataset useful
microscopic-level relationships, relating key local dynamical events (e.g.,
LENS fluctuations) occurring in the systems to the local structural (SOAP)
environments they originate from. Given its abstract nature, we believe that
such an approach will be useful in revealing hidden microscopic
structure-dynamics relationships fundamental to rationalize the behavior of a
variety of complex systems, not necessarily limited to the atomistic and
molecular scales
Gaussian Approximation Potentials: the accuracy of quantum mechanics, without the electrons
We introduce a class of interatomic potential models that can be
automatically generated from data consisting of the energies and forces
experienced by atoms, derived from quantum mechanical calculations. The
resulting model does not have a fixed functional form and hence is capable of
modeling complex potential energy landscapes. It is systematically improvable
with more data. We apply the method to bulk carbon, silicon and germanium and
test it by calculating properties of the crystals at high temperatures. Using
the interatomic potential to generate the long molecular dynamics trajectories
required for such calculations saves orders of magnitude in computational cost.Comment: v3-4: added new material and reference
Gaussian Process Regression for Materials and Molecules.
We provide an introduction to Gaussian process regression (GPR) machine-learning methods in computational materials science and chemistry. The focus of the present review is on the regression of atomistic properties: in particular, on the construction of interatomic potentials, or force fields, in the Gaussian Approximation Potential (GAP) framework; beyond this, we also discuss the fitting of arbitrary scalar, vectorial, and tensorial quantities. Methodological aspects of reference data generation, representation, and regression, as well as the question of how a data-driven model may be validated, are reviewed and critically discussed. A survey of applications to a variety of research questions in chemistry and materials science illustrates the rapid growth in the field. A vision is outlined for the development of the methodology in the years to come
Improved tensor-product expansions for the two-particle density matrix
We present a new density-matrix functional within the recently introduced
framework for tensor-product expansions of the two-particle density matrix. It
performs well both for the homogeneous electron gas as well as atoms. For the
homogeneous electron gas, it performs significantly better than all previous
density-matrix functionals, becoming very accurate for high densities and
outperforming Hartree-Fock at metallic valence electron densities. For isolated
atoms and ions, it is on a par with previous density-matrix functionals and
generalized gradient approximations to density-functional theory. We also
present analytic results for the correlation energy in the low density limit of
the free electron gas for a broad class of such functionals.Comment: 4 pages, 2 figure
A universal preconditioner for simulating condensed phase materials.
We introduce a universal sparse preconditioner that accelerates geometry optimisation and saddle point search tasks that are common in the atomic scale simulation of materials. Our preconditioner is based on the neighbourhood structure and we demonstrate the gain in computational efficiency in a wide range of materials that include metals, insulators, and molecular solids. The simple structure of the preconditioner means that the gains can be realised in practice not only when using expensive electronic structure models but also for fast empirical potentials. Even for relatively small systems of a few hundred atoms, we observe speedups of a factor of two or more, and the gain grows with system size. An open source Python implementation within the Atomic Simulation Environment is available, offering interfaces to a wide range of atomistic codes
Presence of Many Stable Nonhomogeneous States in an Inertial Car-Following Model
A new single lane car following model of traffic flow is presented. The model
is inertial and free of collisions. It demonstrates experimentally observed
features of traffic flow such as the existence of three regimes: free,
fluctuative (synchronized) and congested (jammed) flow; bistability of free and
fluctuative states in a certain range of densities, which causes the hysteresis
in transitions between these states; jumps in the density-flux plane in the
fluctuative regime and gradual spatial transition from synchronized to free
flow. Our model suggests that in the fluctuative regime there exist many stable
states with different wavelengths, and that the velocity fluctuations in the
congested flow regime decay approximately according to a power law in time.Comment: 4 pages, 4 figure
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