1,490 research outputs found
Symmetry-adapted real-space density functional theory for cylindrical geometries: application to large X (X=C, Si, Ge, Sn) nanotubes
We present a symmetry-adapted real-space formulation of Kohn-Sham density
functional theory for cylindrical geometries and apply it to the study of large
X (X=C, Si, Ge, Sn) nanotubes. Specifically, starting from the Kohn-Sham
equations posed on all of space, we reduce the problem to the fundamental
domain by incorporating cyclic and periodic symmetries present in the angular
and axial directions of the cylinder, respectively. We develop a high-order
finite-difference parallel implementation of this formulation, and verify its
accuracy against established planewave and real-space codes. Using this
implementation, we study the band structure and bending properties of X
nanotubes and Xene sheets, respectively. Specifically, we first show that
zigzag and armchair X nanotubes with radii in the range 1 to 5 nm are
semiconducting. In particular, we find an inverse linear dependence of the
bandgap with respect to the radius for all nanotubes, other than the armchair
and zigzag type III carbon variants, for which we find an inverse quadratic
dependence. Next, we exploit the connection between cyclic symmetry and uniform
bending deformations to calculate the bending moduli of Xene sheets in both
zigzag and armchair directions. We find Kirchhoff-Love type bending behavior
for all sheets, with graphene and stanene possessing the largest and smallest
moduli, respectively. In addition, other than graphene, the sheets demonstrate
significant anisotropy, with larger bending moduli along the armchair
direction. Finally, we demonstrate that the proposed approach has very good
parallel scaling and is highly efficient, enabling ab initio simulations of
unprecedented size for systems with a high degree of cyclic symmetry. In
particular, we show that even micron-sized nanotubes can be simulated with
modest computational effort.Comment: 24 pages, 8 figures, 4 table
Periodic Pulay method for robust and efficient convergence acceleration of self-consistent field iterations
Pulay's Direct Inversion in the Iterative Subspace (DIIS) method is one of
the most widely used mixing schemes for accelerating the self-consistent
solution of electronic structure problems. In this work, we propose a simple
generalization of DIIS in which Pulay extrapolation is performed at periodic
intervals rather than on every self-consistent field iteration, and linear
mixing is performed on all other iterations. We demonstrate through numerical
tests on a wide variety of materials systems in the framework of density
functional theory that the proposed generalization of Pulay's method
significantly improves its robustness and efficiency.Comment: Version 2 (with minor edits from version 1
Generalized Emission Functions for Photon Emission from Quark-Gluon Plasma
The Landau-Pomeranchuk-Migdal effects on photon emission from the quark gluon
plasma have been studied as a function of photon mass, at a fixed temperature
of the plasma. The integral equations for the transverse vector function () and the longitudinal function () consisting of multiple scattering effects are solved by the
self consistent iterations method and also by the variational method for the
variable set \{\}, considering the bremsstrahlung and the processes. We define four new dynamical scaling variables,
,,, for bremsstrahlung and {\bf aws} processes and
analyse the transverse and longitudinal components as a function of
\{\}. We generalize the concept of photon emission function and we
define four new emission functions for massive photon emission represented by
, , , . These have been constructed using the exact
numerical solutions of the integral equations. These four emission functions
have been parameterized by suitable simple empirical fits. In terms of these
empirical emission functions, the virtual photon emission from quark gluon
plasma reduces to one dimensional integrals that involve folding over the
empirical functions with appropriate quark distribution
functions and the kinematic factors. Using this empirical emission functions,
we calculated the imaginary part of the photon polarization tensor as a
function of photon mass and energy.Comment: In nuclear physics journals and arxiv listings, my name used to
appear as S.V.S. Sastry. Hereafter, my name will appear as, S.V.
Suryanarayan
Stable Non-BPS States and Their Holographic Duals
Stable non-BPS states can be constructed and studied in a variety of contexts
in string theory. Here we review some interesting constructions that arise from
suspended and wrapped branes. We also exhibit some stable non-BPS states that
have holographic duals.Comment: 10 pages, LaTeX, 10 .eps figures (included); based on a talk given at
Strings 2000, Michiga
Total cross sections for neutron-nucleus scattering
Systematics of neutron scattering cross sections on various materials for
neutron energies up to several hundred MeV are important for ADSS applications.
Ramsauer model is well known and widely applied to understand systematics of
neutron nucleus total cross sections. In this work, we examined the role of
nuclear effective radius parameter (r) on Ramsauer model fits of neutron
total cross sections. We performed Ramsauer model global analysis of the
experimental neutron total cross sections reported by W. P. Abfalterer, F. B.
Bateman, {\it et. al.,}, from 20MeV to 550MeV for nuclei ranging from Be to U .
The global fit functions which can fit total cross section data over periodic
table are provided along with the required global set of parameters. The global
fits predict within deviation to data, showing the scope for
improvement. It has been observed that a finer adjustment of r parameter
alone can give very good Ramsauer model description of neutron total scattering
data within deviation. The required r values for Ramsauer model
fits are shown as a function of nuclear mass number and an empirical formula is
suggested for r values as a function of mass number. In optical model
approach for neutron scattering, we have modified the real part of
Koning-Deleroche potentails to fit the neutron total cross sections using SCAT2
code. The modified potentails have a different energy dependence beyond 200MeV
of neutron energy and fit the total cross sections from Al to Pb.Comment: 9 pages, 20figures, Poster number ND-1457, ND2010 Conference in Kore
Two-level Chebyshev filter based complementary subspace method: pushing the envelope of large-scale electronic structure calculations
We describe a novel iterative strategy for Kohn-Sham density functional
theory calculations aimed at large systems (> 1000 electrons), applicable to
metals and insulators alike. In lieu of explicit diagonalization of the
Kohn-Sham Hamiltonian on every self-consistent field (SCF) iteration, we employ
a two-level Chebyshev polynomial filter based complementary subspace strategy
to: 1) compute a set of vectors that span the occupied subspace of the
Hamiltonian; 2) reduce subspace diagonalization to just partially occupied
states; and 3) obtain those states in an efficient, scalable manner via an
inner Chebyshev-filter iteration. By reducing the necessary computation to just
partially occupied states, and obtaining these through an inner Chebyshev
iteration, our approach reduces the cost of large metallic calculations
significantly, while eliminating subspace diagonalization for insulating
systems altogether. We describe the implementation of the method within the
framework of the Discontinuous Galerkin (DG) electronic structure method and
show that this results in a computational scheme that can effectively tackle
bulk and nano systems containing tens of thousands of electrons, with chemical
accuracy, within a few minutes or less of wall clock time per SCF iteration on
large-scale computing platforms. We anticipate that our method will be
instrumental in pushing the envelope of large-scale ab initio molecular
dynamics. As a demonstration of this, we simulate a bulk silicon system
containing 8,000 atoms at finite temperature, and obtain an average SCF step
wall time of 51 seconds on 34,560 processors; thus allowing us to carry out 1.0
ps of ab initio molecular dynamics in approximately 28 hours (of wall time).Comment: Resubmitted version (version 2
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