487 research outputs found
Efficient and accurate calculation of exact exchange and RPA correlation energies in the Adiabatic-Connection Fluctuation-Dissipation theory
Recently there has been a renewed interest in the calculation of
exact-exchange and RPA correlation energies for realistic systems. These
quantities are main ingredients of the so-called EXX/RPA+ scheme which has been
shown to be a promising alternative approach to the standard LDA/GGA DFT for
weakly bound systems where LDA and GGA perform poorly. In this paper, we
present an efficient approach to compute the RPA correlation energy in the
framework of the Adiabatic-Connection Fluctuation-Dissipation formalism. The
method is based on the calculation of a relatively small number of eigenmodes
of RPA dielectric matrix, efficiently computed by iterative density response
calculations in the framework of Density Functional Perturbation Theory. We
will also discuss a careful treatment of the integrable divergence in the
exact-exchange energy calculation which alleviates the problem of its slow
convergence with respect to Brillouin zone sampling. As an illustration of the
method, we show the results of applications to bulk Si, Be dimer and atomic
systems.Comment: 12 pages, 6 figures. To appear in Phys. Rev.
A comparative study of numerical methods for the overlap Dirac operator--a status report
Improvements of various methods to compute the sign function of the hermitian
Wilson-Dirac matrix within the overlap operator are presented. An optimal
partial fraction expansion (PFE) based on a theorem of Zolotarev is given.
Benchmarks show that this PFE together with removal of converged systems within
a multi-shift CG appears to approximate the sign function times a vector most
efficiently. A posteriori error bounds are given.Comment: 3 pages, poster contribution to Lattice2001(algorithms
Large-scale electronic structure theory for simulating nanostructure process
Fundamental theories and practical methods for large-scale electronic
structure calculations are given, in which the computational cost is
proportional to the system size. Accuracy controlling methods for microscopic
freedoms are focused on two practical solver methods, Krylov-subspace method
and generalized-Wannier-state method. A general theory called the
'multi-solver' scheme is also formulated, as a hybrid between different solver
methods. Practical examples are carried out in several insulating and metallic
systems with 10^3-10^5 atoms. All the theories provide general guiding
principles of constructing an optimal calculation for simulating nanostructure
processes, since a nanostructured system consists of several competitive
regions, such as bulk and surface regions, and the simulation is designed to
reproduce the competition with an optimal computational cost.Comment: 19 pages, 6 figures. To appear in J. Phys. Cond. Matt. A preprint PDF
file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
Conjugate gradient heatbath for ill-conditioned actions
We present a method for performing sampling from a Boltzmann distribution of
an ill-conditioned quadratic action. This method is based on heatbath
thermalization along a set of conjugate directions, generated via a
conjugate-gradient procedure. The resulting scheme outperforms local updates
for matrices with very high condition number, since it avoids the slowing down
of modes with lower eigenvalue, and has some advantages over the global
heatbath approach, compared to which it is more stable and allows for more
freedom in devising case-specific optimizations
Reducing liver lesion incidence in the Dutch pork supply chain
Livers with lesions are an cmportant quality aspect among slaughter pig producers and slaughterhouses. Total losses of non-marketable livers with lesions, lower growth and higher feed intake of pigs in the Netherlands in 2003 were estimated at €3.5 million. The major cause of liver lesions is the roundworm Ascaris suum. Worm treatment on the farm can be effective in reducing liver lesions. Before July 2004 an insurance with a fixed premium for each slaughtered pig was in place in the Netherlands to compensate slaughterhouses for pathological lesions. Individual pig producers had low incentcves to take control measures. In July 2004 a new incentive mechanism was introduced: a reduction in the payment of €1 for each pig with a liver lesion. Thcs placed the financcal burden of levers with lesions on the producer, thereby increasing incentives to treat roundworm infections. We analysed the data of 1,104 farms wcth 55,802 deliveries from 2003 to 2006. The mean liver lesion incidence decreased from 8% in 2003 when a collectcve insurance was in place to 5% in 2006, after the change to the price reduction. Of the producers, 68% reduced liver lesion mcidence. Of the producers with an increased incidence, 83% showed a low increase (less than 5%). We conclude that the price reduction was effective in reducing the mean incidence of liver lesions, although large differences between individual producers exist. Further research is needed to determme what causes these large differences
Linear Algebraic Calculation of Green's function for Large-Scale Electronic Structure Theory
A linear algebraic method named the shifted
conjugate-orthogonal-conjugate-gradient method is introduced for large-scale
electronic structure calculation. The method gives an iterative solver
algorithm of the Green's function and the density matrix without calculating
eigenstates.The problem is reduced to independent linear equations at many
energy points and the calculation is actually carried out only for a single
energy point. The method is robust against the round-off error and the
calculation can reach the machine accuracy. With the observation of residual
vectors, the accuracy can be controlled, microscopically, independently for
each element of the Green's function, and dynamically, at each step in
dynamical simulations. The method is applied to both semiconductor and metal.Comment: 10 pages, 9 figures. To appear in Phys. Rev. B. A PDF file with
better graphics is available at http://fujimac.t.u-tokyo.ac.jp/lses
Lattice field theory simulations of graphene
We discuss the Monte Carlo method of simulating lattice field theories as a
means of studying the low-energy effective theory of graphene. We also report
on simulational results obtained using the Metropolis and Hybrid Monte Carlo
methods for the chiral condensate, which is the order parameter for the
semimetal-insulator transition in graphene, induced by the Coulomb interaction
between the massless electronic quasiparticles. The critical coupling and the
associated exponents of this transition are determined by means of the
logarithmic derivative of the chiral condensate and an equation-of-state
analysis. A thorough discussion of finite-size effects is given, along with
several tests of our calculational framework. These results strengthen the case
for an insulating phase in suspended graphene, and indicate that the
semimetal-insulator transition is likely to be of second order, though
exhibiting neither classical critical exponents, nor the predicted phenomenon
of Miransky scaling.Comment: 14 pages, 7 figures. Published version freely available if accessed
via http://physics.aps.org/articles/v2/3
An Improvement of Davidson's Iteration Method: Applications to MRCI and MRCEPA Calculations
ABSTRAC
Parallel Sparse LU Decomposition on a Mesh Network of Transputers
A parallel algorithm is presented for the LU decomposition of a general sparse matrix on a distributed-memory MIMD multiprocessor with a square mesh communication network. In the algorithm, matrix elements are assigned to processors according to the grid distribution. Each processor represents the nonzero elements of its part of the matrix by a local, ordered, two-dimensional linked-list data structure. The complexity of important operations on this data structure and on several others is analysed. At each step of the algorithm, a parallel search for a set of m compatible pivot elements is performed. The Markowitz counts of the pivot elements are close to minimum, to preserve the sparsity of the matrix. The pivot elements also satisfy a threshold criterion, to ensure numerical stability. The compatibility of the m pivots enables the simultaneous elimination of m pivot rows and m pivot columns in a rank-m update of the reduced matrix. Experimental results on a network of 400 transputers are presented for a set of test matrices from the Harwell–Boeing sparse matrix collection
Improved Quenched QCD on Large Lattices - First Results
Continuing our investigations of quenched QCD with improved fermions we have
started simulations for lattice size 32^3 x 64 at beta=6.2. We present first
results for light hadron masses at kappa=0.13520, 0.13540, and 0.13555.
Moreover we compare our initial experiences on the T3E with those for
APE/Quadrics systems.Comment: 3 pages, Latex2e, 4 figures, espcrc2, epsfig and latexsym require
- …