8 research outputs found
Cyclic Density Functional Theory : A route to the first principles simulation of bending in nanostructures
We formulate and implement Cyclic Density Functional Theory (Cyclic DFT) -- a
self-consistent first principles simulation method for nanostructures with
cyclic symmetries. Using arguments based on Group Representation Theory, we
rigorously demonstrate that the Kohn-Sham eigenvalue problem for such systems
can be reduced to a fundamental domain (or cyclic unit cell) augmented with
cyclic-Bloch boundary conditions. Analogously, the equations of electrostatics
appearing in Kohn-Sham theory can be reduced to the fundamental domain
augmented with cyclic boundary conditions. By making use of this symmetry cell
reduction, we show that the electronic ground-state energy and the
Hellmann-Feynman forces on the atoms can be calculated using quantities defined
over the fundamental domain. We develop a symmetry-adapted finite-difference
discretization scheme to obtain a fully functional numerical realization of the
proposed approach. We verify that our formulation and implementation of Cyclic
DFT is both accurate and efficient through selected examples.
The connection of cyclic symmetries with uniform bending deformations
provides an elegant route to the ab-initio study of bending in nanostructures
using Cyclic DFT. As a demonstration of this capability, we simulate the
uniform bending of a silicene nanoribbon and obtain its energy-curvature
relationship from first principles. A self-consistent ab-initio simulation of
this nature is unprecedented and well outside the scope of any other systematic
first principles method in existence. Our simulations reveal that the bending
stiffness of the silicene nanoribbon is intermediate between that of graphene
and molybdenum disulphide. We describe several future avenues and applications
of Cyclic DFT, including its extension to the study of non-uniform bending
deformations and its possible use in the study of the nanoscale flexoelectric
effect.Comment: Version 3 of the manuscript, Accepted for publication in Journal of
the Mechanics and Physics of Solids,
http://www.sciencedirect.com/science/article/pii/S002250961630368
STRESS TENSOR IN REAL-SPACE KOHN-SHAM DENSITY FUNCTIONAL THEORY
An accurate and efficient formulation of the stress tensor for real-space Kohn-ShamDensity Functional Theory (DFT) calculations is presented. Specifically, while employinga local formulation of the electrostatics, a linear-scaling expression for the stress tensorthat is applicable to simulations with unit cells of arbitrary symmetry, semilocal exchange-correlation functionals, and Brillouin zone integration is discussed. In particular, the contri-butions arising from the self energy and the nonlocal pseudopotential energy are rewrittento make them suitable for the real-space finite-difference discretization, achieving up tothree orders of magnitude improvement in the accuracy of the computed stresses. Throughselected examples representative of static calculations, the accuracy and efficiency of theproposed formulation is verified. In particular, high rates of convergence with spatial dis-cretization, consistency between the computed energy and stress tensor, and very goodagreement with reference planewave results are demonstrated.M.S
Density functional theory method for twisted geometries with application to torsional deformations in group-IV nanotubes
We present a real-space formulation and implementation of Kohn-Sham Density
Functional Theory suited to twisted geometries, and apply it to the study of
torsional deformations of X (X = C, Si, Ge, Sn) nanotubes. Our formulation is
based on higher order finite difference discretization in helical coordinates,
uses ab intio pseudopotentials, and naturally incorporates rotational (cyclic)
and screw operation (i.e., helical) symmetries. We discuss several aspects of
the computational method, including the form of the governing equations,
details of the numerical implementation, as well as its convergence, accuracy
and efficiency properties.
The technique presented here is particularly well suited to the first
principles simulation of quasi-one-dimensional structures and their
deformations, and many systems of interest can be investigated using small
simulation cells containing just a few atoms. We apply the method to
systematically study the properties of single-wall zigzag and armchair group-IV
nanotubes, as they undergo twisting. For the range of deformations considered,
the mechanical behavior of the tubes is found to be largely consistent with
isotropic linear elasticity, with the torsional stiffness varying as the cube
of the nanotube radius. Furthermore, for a given tube radius, this quantity is
seen to be highest for carbon nanotubes and the lowest for those of tin, while
nanotubes of silicon and germanium have intermediate values close to each
other. We also describe different aspects of the variation in electronic
properties of the nanotubes as they are twisted. In particular, we find that
akin to the well known behavior of armchair carbon nanotubes, armchair
nanotubes of silicon, germanium and tin also exhibit bandgaps that vary
periodically with imposed rate of twist, and that the periodicity of the
variation scales in an inverse quadratic manner with the tube radius
Ab initio framework for systems with helical symmetry: theory, numerical implementation and applications to torsional deformations in nanostructures
We formulate and implement Helical DFT -- a self-consistent first principles
simulation method for nanostructures with helical symmetries. Such materials
are well represented in all of nanotechnology, chemistry and biology, and are
expected to be associated with unprecedented material properties. We rigorously
demonstrate the existence and completeness of special solutions to the single
electron problem for helical nanostructures, called helical Bloch waves. We
describe how the Kohn-Sham Density Functional Theory equations for a helical
nanostructure can be reduced to a fundamental domain with the aid of these
solutions. A key component in our mathematical treatment is the definition and
use of a helical Bloch-Floquet transform to perform a block-diagonalization of
the Hamiltonian in the sense of direct integrals. We develop a symmetry-adapted
finite-difference strategy in helical coordinates to discretize the governing
equations, and obtain a working realization of the proposed approach. We verify
the accuracy and convergence properties of our numerical implementation through
examples. Finally, we employ Helical DFT to study the properties of zigzag and
chiral single wall black phosphorus (i.e., phosphorene) nanotubes. We use our
simulations to evaluate the torsional stiffness of a zigzag nanotube ab initio.
Additionally, we observe an insulator-to-metal-like transition in the
electronic properties of this nanotube as it is subjected to twisting. We also
find that a similar transition can be effected in chiral phosphorene nanotubes
by means of axial strains. Notably, self-consistent ab initio simulations of
this nature are unprecedented and well outside the scope of any other
systematic first principles method in existence. We end with a discussion on
various future avenues and applications
Algorithmes incrémentaux pour la théorie de la fonctionnelle de la densité
The ability to model molecular systems on a computer has become a crucial tool for chemists. Indeed molecular simulations have helped to understand and predict properties of nanoscopic world, and during the last decades have had large impact on domains like biology, electronic or materials development. Particle simulation is a classical method of molecular dynamic. In particle simulation, molecules are split into atoms, their inter-atomic interactions are computed, and their time trajectories are derived step by step. Unfortunately, inter-atomic interactions computation costs prevent large systems to be modeled in a reasonable time. In this context, our research team looks for new accurate and efficient molecular simulation models. One of our team's focus is the search and elimination of useless calculus in dynamical simulations. Hence has been proposed a new adaptively restrained dynamical model in which the slowest particles movement is frozen, computational time is saved if the interaction calculus method do not compute again interactions between static atoms. The team also developed several interaction models that benefit from a restrained dynamical model, they often updates interactions incrementally using the previous time step results and the knowledge of which particle have moved.In the wake of our team's work, we propose in this thesis an incremental First-principles interaction models. Precisely, we have developed an incremental Orbital-Free Density Functional Theory method that benefits from an adaptively restrained dynamical model. The new OF-DFT model keeps computation in Real-Space, so can adaptively focus computations where they are necessary. The method is first proof-tested, then we show its ability to speed up computations when a majority of particle are static and with a restrained particle dynamic model. This work is a first step toward a combination of incremental First-principle interaction models and adaptively restrained particle dynamic models.In the wake of our team's work, we propose in this thesis an incremental First-principles interaction models. Precisely, we have developed an incremental Orbital-Free Density Functional Theory method that benefits from an adaptively restrained dynamical model. The new OF-DFT model keeps computation in Real-Space, so can adaptively focus computations where they are necessary. The method is first proof-tested, then we show its ability to speed up computations when a majority of particle are static and with a restrained particle dynamic model. This work is a first step toward a combination of incremental First-principle interaction models and adaptively restrained particle dynamic models.L'informatique est devenue un outil incontournable de la chimie. En effet la capacité de simuler des molécules sur ordinateur a aidé à la compréhension du monde nanoscopic et à la prédiction de ses propriétés. La simulation moléculaire a eu ces dernières décennies un impact scientifique énorme en biologie, en électronique, en science des matériaux ... La simulation de particules est une des méthodes classiques de dynamique moléculaire, les molécules y sont divisées en atomes, leurs interactions relatives calculées et leurs trajectoires déduites pas à pas. Malheureusement un calcul précis des interactions entre atomes demande énormément d'opérations et donc de temps, ce qui limite la portée de la simulation moléculaire à des systèmes de taille raisonnable. C'est dans ce contexte que notre équipe recherche de nouveaux modèles de simulation moléculaire rapide et précis. Un des angles de recherche est l'élimination des calculs inutiles des simulations. L'équipe a ainsi proposé un modèle de dynamique moléculaire dite restreinte de manière adaptative dans lequel le mouvement des particules les plus lentes est bloqué. Si la simulation ne recalcule pas les interactions inchangées entre atomes bloqués, le calcul des interactions est plus rapide. L'équipe a aussi développé plusieurs modèles d'interactions plus efficaces pour des modèles de dynamique restreinte de particules, ils mettent à jour les interactions de façon incrémentale en utilisant les résultats du pas de temps précédent et la liste des particules mobiles. Dans le sillage des travaux de notre équipe de recherche, nous proposons dans cette thèse une méthode incrémentale pour calculer des interactions interatomique basées sur les modèles de Théorie de la Fonctionnelle de la Densité Sans Orbitale. La nouvelle méthode garde les calculs dans l'espace réel et peut ainsi concentrer les calculs où cela est nécessaire. Dans ce manuscrit nous vérifions cette méthode, puis nous évaluons les gains de vitesse lorsqu'une majorité de particule est bloquée, avec un modèle de dynamique restreinte. Ces travaux sont un pas vers la l'intégration de modèles d'interactions Premier-principes pour des modèles dynamiques restreint de manière adaptative