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Thomas Kalkreuter

Publications and source records attributed to Thomas Kalkreuter.

11 recordsLinked to original sources
arXiv1995

Numerical analysis of the spectrum of the Dirac operator in four-dimensional SU(2) gauge fields

Two numerical algorithms for the computation of eigenvalues of Dirac operators in lattice gauge theories are described: one is an accelerated conjugate gradient method, the other one a standard Lanczos method. Results obtained by Cullum's and Willoughby's variant of the Lanczos method (whose convergence behaviour is closely linked with the local spectral density) are presented for euclidean Wilson fermions in quenched and unquenched SU(2) gauge fields. Complete spectra are determined on lattices up to $8^3 \cdot 12$, and we derive numerical values for fermionic determinants and results for spectral densities.

hep-lat↗
arXiv1995

Study of Cullum's and Willoughby's Lanczos method for Wilson fermions

The Lanczos method of Cullum and Willoughby is studied for euclidean Wilson fermions in quenched and unquenched SU(2) gauge fields on lattices of volume ranging from $4^4$ to $16^4$. The method is reliable even on larger lattices, but its cost for the computation of a given fraction of the spectrum grows (approximately) with the square of the lattice volume. We investigate the convergence behaviour and show that it is closely linked with the local spectral density. Complete spectra are determined on lattices up to $8^3 \cdot 12$. For configurations where all eigenvalues are computed, we give numerical values for the fermionic determinants and results for spectral densities. Determinants are also given for staggered fermions whose quenched and unquenched spectra were studied in a previous publication.

hep-lat↗
arXiv1995

An Accelerated Conjugate Gradient Algorithm to Compute Low-Lying Eigenvalues --- a Study for the Dirac Operator in SU(2) Lattice QCD

The low-lying eigenvalues of a (sparse) hermitian matrix can be computed with controlled numerical errors by a conjugate gradient (CG) method. This CG algorithm is accelerated by alternating it with exact diagonalisations in the subspace spanned by the numerically computed eigenvectors. We study this combined algorithm in case of the Dirac operator with (dynamical) Wilson fermions in four-dimensional $\SUtwo$ gauge fields. The algorithm is numerically very stable and can be parallelized in an efficient way. On lattices of sizes $4^4-16^4$ an acceleration of the pure CG method by a factor of~$4-8$ is found.

hep-lat↗
arXiv1994

Multigrid Methods for Propagators in Lattice Gauge Theories

Multigrid methods were invented for the solution of discretized partial differential equations in ordered systems. The slowness of traditional algorithms is overcome by updates on various length scales. In this article we discuss generalizations of multigrid methods for disordered systems, in particular for propagators in lattice gauge theories. A discretized nonabelian gauge theory can be formulated as a system of statistical mechanics where the gauge field degrees of freedom are $SU(N)$ matrices on the links of the lattice. These $SU(N)$ matrices appear as random coefficients in Dirac equations. We aim at finding an efficient method by which one can solve Dirac equations without critical slowing down. If this could be achieved, Monte Carlo simulations of Quantum Chromodynamics (the theory of the strong interaction) would be accelerated considerably. In principle, however, the methods discussed can be used in arbitrary space-time dimension and for arbitrary gauge group. Moreover, there are applications in multigrid Monte Carlo simulations, and for the definition of block spins and blocked gauge fields in Monte Carlo renormalization group studies. As a central results it was found that {\em the geometric multigrid method works in principle in arbitrarily disordered gauge fields.} Finally, an overview is given of other approaches to the propagator problem in lattice gauge theories.

hep-lat↗
arXiv1994

Spectrum of the Dirac Operator and Multigrid Algorithm with Dynamical Staggered Fermions

Complete spectra of the staggered Dirac operator $\Dirac$ are determined in quenched four-dimensional $SU(2)$ gauge fields, and also in the presence of dynamical fermions. Periodic as well as antiperiodic boundary conditions are used. An attempt is made to relate the performance of multigrid (MG) and conjugate gradient (CG) algorithms for propagators with the distribution of the eigenvalues of~$\Dirac$. The convergence of the CG algorithm is determined only by the condition number~$κ$ and by the lattice size. Since~$κ$'s do not vary significantly when quarks become dynamic, CG convergence in unquenched fields can be predicted from quenched simulations. On the other hand, MG convergence is not affected by~$κ$ but depends on the spectrum in a more subtle way.

hep-lat↗
arXiv1993

Idealized Multigrid Algorithm for Staggered Fermions

An idealized multigrid algorithm for the computation of propagators of staggered fermions is investigated. Exemplified in four-dimensional $SU(2)$ gauge fields, it is shown that the idealized algorithm preserves criticality under coarsening. The same is not true when the coarse grid operator is defined by the Galerkin prescription. Relaxation times in computations of propagators are small, and critical slowing is strongly reduced (or eliminated) in the idealized algorithm. Unfortunately, this algorithm is not practical for production runs, but the investigations presented here answer important questions of principle.

hep-lat↗
arXiv1992

Multigrid Methods for the Computation of Propagators in Gauge Fields

NOTE: this is a shortened version of the abstract of the paper. Multigrid methods for propagators in gauge fields are investigated. Gauge fields are incorporated in algorithms in a covariant way. This avoids the necessity for gauge fixing in computations of propagators. The kernel $C$ of the restriction operator which averages from one grid to the next coarser grid is defined by projection on the ground- state of a local Hamiltonian. The idea behind this definition is that the appropriate notion of smoothness depends on the dynamics. The ground-state projection choice of $C$ is usable in arbitrary space-time dimension $d$ and for arbitrary gauge group. We discuss proper averaging operations for bosons and for staggered fermions. The averaging kernels $C$ can be used not only in deterministic multigrid computations, but also in multigrid Monte Carlo simulations, and for the definition of block spins and blocked gauge fields in Monte Carlo renormalization group studies of gauge theories. Actual numerical computations of kernels and propagators are performed in compact four-dimensional $SU(2)$ gauge fields. A central result of the present work is that {\em the multigrid method works in arbitrarily disordered gauge fields, in principle\/}.

hep-lat↗
arXiv1992

Multigrid for propagators of staggered fermions in four-dimensional $SU(2)$ gauge fields

Multigrid (MG) methods for the computation of propagators of staggered fermions in non-Abelian gauge fields are discussed. MG could work in principle in arbitrarily disordered systems. The practical variational MG methods tested so far with a ``Laplacian choice'' for the restriction operator are not competitive with the conjugate gradient algorithm on lattices up to $18^4$. Numerical results are presented for propagators in $SU(2)$ gauge fields.

hep-lat↗
arXiv1992

Improving Multigrid and Conventional Relaxation Algorithms for Propagators

Practical modifications of deterministic multigrid and conventional relaxation algorithms are discussed. New parameters need not be tuned but are determined by the algorithms themselves. One modification can be thought of as ``updating on a last layer consisting of a single site''. It eliminates critical slowing down in computations of bosonic and fermionic propagators in a fixed volume. Here critical slowing down means divergence of asymptotic relaxation times as the propagators approach criticality. A remaining volume dependence is weak enough in case of bosons so that conjugate gradient can be outperformed. However, no answer can be given yet if the same is true for staggered fermions on lattices of realizable sizes. Numerical results are presented for propagators of bosons and of staggered fermions in 4-dimensional $SU(2)$ gauge fields.

hep-lat↗