arXiv subjects

Jacques C. R. Bloch

Publications and source records attributed to Jacques C. R. Bloch.

7 recordsLinked to original sources

arXiv2011

Evading the sign problem in random matrix simulations

In this talk we show how the sign problem, occurring in dynamical simulations of random matrices at nonzero chemical potential, can be avoided by judiciously combining matrices into subsets. One can prove that these subsets have real and positive weights such that importance sampling can be used in Monte Carlo simulations. The number of matrices per subset is proportional to the matrix dimension. We measure the chiral condensate and observe that the statistical error is independent of the chemical potential and grows linearly with the matrix dimension, which contrasts strongly with its exponential growth in reweighting methods.

hep-lat↗

arXiv2011

A nested Krylov subspace method to compute the sign function of large complex matrices

We present an acceleration of the well-established Krylov-Ritz methods to compute the sign function of large complex matrices, as needed in lattice QCD simulations involving the overlap Dirac operator at both zero and nonzero baryon density. Krylov-Ritz methods approximate the sign function using a projection on a Krylov subspace. To achieve a high accuracy this subspace must be taken quite large, which makes the method too costly. The new idea is to make a further projection on an even smaller, nested Krylov subspace. If additionally an intermediate preconditioning step is applied, this projection can be performed without affecting the accuracy of the approximation, and a substantial gain in efficiency is achieved for both Hermitian and non-Hermitian matrices. The numerical efficiency of the method is demonstrated on lattice configurations of sizes ranging from 4^4 to 10^4, and the new results are compared with those obtained with rational approximation methods.

hep-lat↗

arXiv2010

Short-recurrence Krylov subspace methods for the overlap Dirac operator at nonzero chemical potential

The overlap operator in lattice QCD requires the computation of the sign function of a matrix, which is non-Hermitian in the presence of a quark chemical potential. In previous work we introduced an Arnoldi-based Krylov subspace approximation, which uses long recurrences. Even after the deflation of critical eigenvalues, the low efficiency of the method restricts its application to small lattices. Here we propose new short-recurrence methods which strongly enhance the efficiency of the computational method. Using rational approximations to the sign function we introduce two variants, based on the restarted Arnoldi process and on the two-sided Lanczos method, respectively, which become very efficient when combined with multishift solvers. Alternatively, in the variant based on the two-sided Lanczos method the sign function can be evaluated directly. We present numerical results which compare the efficiencies of a restarted Arnoldi-based method and the direct two-sided Lanczos approximation for various lattice sizes. We also show that our new methods gain substantially when combined with deflation.

hep-lat↗

arXiv2009

A nested Krylov subspace method for the overlap operator

We present a novel method to compute the overlap Dirac operator at zero and nonzero quark chemical potential. To approximate the sign function of large, sparse matrices, standard methods project the operator on a much smaller Krylov subspace, on which the matrix function is computed exactly. However, for large lattices this subspace can still be too large for an efficient calculation of the sign function. The idea of the new method is to nest Krylov subspace approximations by making a further projection on an even smaller subspace, which is then small enough to compute the sign function efficiently, and this without any noticeable loss of numerical accuracy. We demonstrate the efficiency of the method both on Hermitian and non-Hermitian matrices.

hep-lat↗

arXiv2009

Krylov subspace methods and the sign function: multishifts and deflation in the non-Hermitian case

Rational approximations of the matrix sign function lead to multishift methods. For non-Hermitian matrices long recurrences can cause storage problems, which can be circumvented with restarts. Together with deflation we obtain efficient iterative methods, as we show in numerical experiments for the overlap Dirac operator at non-vanishing quark chemical potential for lattices up to size 10^4.

hep-lat↗

arXiv2003

Propagators and running coupling from SU(2) lattice gauge theory

We perform numerical studies of the running coupling constant alpha_R(p^2) and of the gluon and ghost propagators for pure SU(2) lattice gauge theory in the minimal Landau gauge. Different definitions of the gauge fields and different gauge-fixing procedures are used respectively for gaining better control over the approach to the continuum limit and for a better understanding of Gribov-copy effects. We find that the ghost-ghost-gluon-vertex renormalization constant is finite in the continuum limit, confirming earlier results by all-order perturbation theory. In the low momentum regime, the gluon form factor is suppressed while the ghost form factor is divergent. Correspondingly, the ghost propagator diverges faster than 1/p^2 and the gluon propagator appears to be finite. Precision data for the running coupling alpha_R(p^2) are obtained. These data are consistent with an IR fixed point given by lim_{p \to 0} alpha_R(p^2) = 5(1).

hep-lat↗

arXiv2002

Running coupling constant and propagators in SU(2) Landau gauge

We present a numerical study of the running coupling constant and of the gluon and ghost propagators in minimal Landau gauge. Simulations are done in pure SU(2) lattice gauge theory for several values of beta and lattice sizes. We use two different lattice setups.

hep-lat↗