10,655 research outputs found
Edge reconstruction in armchair phosphorene nanoribbons revealed by discontinuous Galerkin density functional theory
With the help of our recently developed massively parallel DGDFT
(Discontinuous Galerkin Density Functional Theory) methodology, we perform
large-scale Kohn-Sham density functional theory calculations on phosphorene
nanoribbons with armchair edges (ACPNRs) containing a few thousands to ten
thousand atoms. The use of DGDFT allows us to systematically achieve
conventional plane wave basis set type of accuracy, but with a much smaller
number (about 15) of adaptive local basis (ALB) functions per atom for this
system. The relatively small number degrees of freedom required to represent
the Kohn-Sham Hamiltonian, together with the use of the pole expansion the
selected inversion (PEXSI) technique that circumvents the need to diagonalize
the Hamiltonian, result in a highly efficient and scalable computational scheme
for analyzing the electronic structures of ACPNRs as well as its dynamics. The
total wall clock time for calculating the electronic structures of large-scale
ACPNRs containing 1080-10800 atoms is only 10-25 s per self-consistent field
(SCF) iteration, with accuracy fully comparable to that obtained from
conventional planewave DFT calculations. For the ACPNR system, we observe that
the DGDFT methodology can scale to 5,000-50,000 processors. We use DGDFT based
ab-initio molecular dynamics (AIMD) calculations to study the thermodynamic
stability of ACPNRs. Our calculations reveal that a 2 * 1 edge reconstruction
appears in ACPNRs at room temperature.Comment: 9 pages, 5 figure
Projected Commutator DIIS Method for Accelerating Hybrid Functional Electronic Structure Calculations
The commutator direct inversion of the iterative subspace (commutator DIIS or
C-DIIS) method developed by Pulay is an efficient and the most widely used
scheme in quantum chemistry to accelerate the convergence of self consistent
field (SCF) iterations in Hartree-Fock theory and Kohn-Sham density functional
theory. The C-DIIS method requires the explicit storage of the density matrix,
the Fock matrix and the commutator matrix. Hence the method can only be used
for systems with a relatively small basis set, such as the Gaussian basis set.
We develop a new method that enables the C-DIIS method to be efficiently
employed in electronic structure calculations with a large basis set such as
planewaves for the first time. The key ingredient is the projection of both the
density matrix and the commutator matrix to an auxiliary matrix called the
gauge-fixing matrix. The resulting projected commutator-DIIS method (PC-DIIS)
only operates on matrices of the same dimension as the that consists of
Kohn-Sham orbitals. The cost of the method is comparable to that of standard
charge mixing schemes used in large basis set calculations. The PC-DIIS method
is gauge-invariant, which guarantees that its performance is invariant with
respect to any unitary transformation of the Kohn-Sham orbitals. We demonstrate
that the PC-DIIS method can be viewed as an extension of an iterative
eigensolver for nonlinear problems. We use the PC-DIIS method for accelerating
Kohn-Sham density functional theory calculations with hybrid
exchange-correlation functionals, and demonstrate its superior performance
compared to the commonly used nested two-level SCF iteration procedure
DGDFT: A Massively Parallel Method for Large Scale Density Functional Theory Calculations
We describe a massively parallel implementation of the recently developed
discontinuous Galerkin density functional theory (DGDFT) [J. Comput. Phys.
2012, 231, 2140] method, for efficient large-scale Kohn-Sham DFT based
electronic structure calculations. The DGDFT method uses adaptive local basis
(ALB) functions generated on-the-fly during the self-consistent field (SCF)
iteration to represent the solution to the Kohn-Sham equations. The use of the
ALB set provides a systematic way to improve the accuracy of the approximation.
It minimizes the number of degrees of freedom required to represent the
solution to the Kohn-Sham problem for a desired level of accuracy. In
particular, DGDFT can reach the planewave accuracy with far fewer numbers of
degrees of freedom. By using the pole expansion and selected inversion (PEXSI)
technique to compute electron density, energy and atomic forces, we can make
the computational complexity of DGDFT scale at most quadratically with respect
to the number of electrons for both insulating and metallic systems. We show
that DGDFT can achieve 80% parallel efficiency on 128,000 high performance
computing cores when it is used to study the electronic structure of
two-dimensional (2D) phosphorene systems with 3,500-14,000 atoms. This high
parallel efficiency results from a two-level parallelization scheme that we
will describe in detail.Comment: 13 pages, 8 figures in J. Chem. Phys. 2015. arXiv admin note: text
overlap with arXiv:1501.0503
Parallel energy-stable phase field crystal simulations based on domain decomposition methods
In this paper, we present a parallel numerical algorithm for solving the
phase field crystal equation. In the algorithm, a semi-implicit finite
difference scheme is derived based on the discrete variational derivative
method. Theoretical analysis is provided to show that the scheme is
unconditionally energy stable and can achieve second-order accuracy in both
space and time. An adaptive time step strategy is adopted such that the time
step size can be flexibly controlled based on the dynamical evolution of the
problem. At each time step, a nonlinear algebraic system is constructed from
the discretization of the phase field crystal equation and solved by a domain
decomposition based, parallel Newton--Krylov--Schwarz method with improved
boundary conditions for subdomain problems. Numerical experiments with several
two and three dimensional test cases show that the proposed algorithm is
second-order accurate in both space and time, energy stable with large time
steps, and highly scalable to over ten thousands processor cores on the Sunway
TaihuLight supercomputer
Electronic Structure of Large-Scale Graphene Nanoflakes
With the help of the recently developed SIESTA-PEXSI method [J. Phys.:
Condens. Matter \textbf{26}, 305503 (2014)], we perform Kohn-Sham density
functional theory (DFT) calculations to study the stability and electronic
structure of hexagonal graphene nanoflakes (GNFs) with up to 11,700 atoms. We
find the electronic properties of GNFs, including their cohesive energy,
HOMO-LUMO energy gap, edge states and aromaticity, depend sensitively on the
type of edges (ACGNFs and ZZGNFs), size and the number of electrons. We observe
that, due to the edge-induced strain effect in ACGNFs, large-scale ACGNFs'
cohesive energy decreases as their size increases. This trend does not hold for
ZZGNFs due to the presence of many edge states in ZZGNFs. We find that the
energy gaps of GNFs all decay with respect to , where is the
size of the GNF, in a linear fashion. But as their size increases, ZZGNFs
exhibit more localized edge states. We believe the presence of these states
makes their gap decrease more rapidly. In particular, when is larger than
6.40 , we find that ZZGNFs exhibit metallic characteristics. Furthermore,
we find that the aromatic structures of GNFs appear to depend only on whether
the system has or electrons, where is an integer.Comment: 11 pages, 9 figure
Ricci-flat graphs with girth four
Lin-Lu-Yau introduced an interesting notion of Ricci curvature for graphs and
obtained a complete characterization for all Ricci-flat graphs with girth at
least five [1]. In this paper, we propose a concrete approach to construct an
infinite family of distinct Ricci-flat graphs of girth four with edge-disjoint
4-cycles and completely characterize all Ricci-flat graphs of girth four with
vertex-disjoint 4-cycles
GCP Code Applied in Production and Absorption
Using the General Cascade Program(GCP), the production and absorption of
in p-A and A-A collisions have been studied. Nucleon absorption
mechanism and comover absorption mechanism are considered to investigate the
suppression. The results agree well with experimental data of
production, except for the data in Pb-Pb collision.Comment: 17 pages, revtex, 6 figure
Quantum Discrete Cosine Transform for Image Compression
Discrete Cosine Transform (DCT) is very important in image compression.
Classical 1-D DCT and 2-D DCT has time complexity O(NlogN) and O(N²logN)
respectively. This paper presents a quantum DCT iteration, and constructs a
quantum 1-D and 2-D DCT algorithm for image compression by using the iteration.
The presented 1-D and 2-D DCT has time complexity O(sqrt(N)) and O(N)
respectively. In addition, the method presented in this paper generalizes the
famous Grover's algorithm to solve complex unstructured search problem.Comment: Modify on Jan. 22, 2006. Only Add ref. 1
Testing the Einstein's equivalence principle with polarized gamma-ray bursts
The Einstein's equivalence principle can be tested by using parameterized
post-Newtonian parameters, of which the parameter has been constrained
by comparing the arrival times of photons with different energies. It has been
constrained by a variety of astronomical transient events, such as gamma-ray
bursts (GRBs), fast radio bursts as well as pulses of pulsars, with the most
stringent constraint of . In this letter, we
consider the arrival times of lights with different circular polarization. For
a linearly polarized light, it is the combination of two circularly polarized
lights. If the arrival time difference between the two circularly polarized
lights is too large, their combination may lose the linear polarization. We
constrain the value of by the
measurement of the polarization of GRB 110721A, which is the most stringent
constraint ever achieved
Wetting and Diffusion of Water on Pristine and Strained Phosphorene
Phosphorene, a newly fabricated two-dimensional (2D) nanomaterial, have
exhibited promising application prospect in biology. Nonetheless, the wetting
and diffusive properties of bio-fluids on phosphorene are still elusive. In
this study, using molecular dynamics (MD) simulations, we investigated the
structural and dynamic properties of water on pristine and strained
phosphorene. The MD simulations illustrated that the diffusion of water
molecules on the phosphorene surface is anisotropic, while strain-enhanced
diffusion is clearly present which arises from strain-induced smooth of the
energy landscape. The contact angle of water droplet on phosphorene exhibited a
nonmonotonic variation with the transverse strain. The structure of water on
transverse stretched phosphorene was demonstrated to be different from that on
longitudinal stretched phosphorene. Moreover, we discovered that the contact
angle of water on strained phosphorene is proportional to the quotient of
longitudinal and transverse diffusion coefficients of interfacial water. These
findings would offer helpful insights in potential ways of manipulating the
wetting and transport of water at nanoscale, and in future bio-applications of
phosphorene.Comment: 8 pages, 6 figure
- …