3,671 research outputs found
Block bond-order potential as a convergent moments-based method
The theory of a novel bond-order potential, which is based on the block
Lanczos algorithm, is presented within an orthogonal tight-binding
representation. The block scheme handles automatically the very different
character of sigma and pi bonds by introducing block elements, which produces
rapid convergence of the energies and forces within insulators, semiconductors,
metals, and molecules. The method gives the first convergent results for
vacancies in semiconductors using a moments-based method with a low number of
moments. Our use of the Lanczos basis simplifies the calculations of the band
energy and forces, which allows the application of the method to the molecular
dynamics simulations of large systems. As an illustration of this convergent
O(N) method we apply the block bond-order potential to the large scale
simulation of the deformation of a carbon nanotube.Comment: revtex, 43 pages, 11 figures, submitted to Phys. Rev.
Linear scaling calculation of band edge states and doped semiconductors
Linear scaling methods provide total energy, but no energy levels and
canonical wavefuctions. From the density matrix computed through the density
matrix purification methods, we propose an order-N (O(N)) method for
calculating both the energies and wavefuctions of band edge states, which are
important for optical properties and chemical reactions. In addition, we also
develop an O(N) algorithm to deal with doped semiconductors based on the O(N)
method for band edge states calculation. We illustrate the O(N) behavior of the
new method by applying it to boron nitride (BN) nanotubes and BN nanotubes with
an adsorbed hydrogen atom. The band gap of various BN nanotubes are
investigated systematicly and the acceptor levels of BN nanotubes with an
isolated adsorbed H atom are computed. Our methods are simple, robust, and
especially suited for the application in self-consistent field electronic
structure theory
Improving the Efficiency of FP-LAPW Calculations
The full-potential linearized augmented-plane wave (FP-LAPW) method is well
known to enable most accurate calculations of the electronic structure and
magnetic properties of crystals and surfaces. The implementation of atomic
forces has greatly increased it's applicability, but it is still generally
believed that FP-LAPW calculations require substantial higher computational
effort compared to the pseudopotential plane wave (PPW) based methods.
In the present paper we analyse the FP-LAPW method from a computational point
of view. Starting from an existing implementation (WIEN95 code), we identified
the time consuming parts and show how some of them can be formulated more
efficiently. In this context also the hardware architecture plays a crucial
role. The remaining computational effort is mainly determined by the setup and
diagonalization of the Hamiltonian matrix. For the latter, two different
iterative schemes are compared. The speed-up gained by these optimizations is
compared to the runtime of the ``original'' version of the code, and the PPW
approach. We expect that the strategies described here, can also be used to
speed up other computer codes, where similar tasks must be performed.Comment: 20 pages, 3 figures. Appears in Comp. Phys. Com. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
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