12,273 research outputs found
Quantum Monte Carlo for large chemical systems: Implementing efficient strategies for petascale platforms and beyond
Various strategies to implement efficiently QMC simulations for large
chemical systems are presented. These include: i.) the introduction of an
efficient algorithm to calculate the computationally expensive Slater matrices.
This novel scheme is based on the use of the highly localized character of
atomic Gaussian basis functions (not the molecular orbitals as usually done),
ii.) the possibility of keeping the memory footprint minimal, iii.) the
important enhancement of single-core performance when efficient optimization
tools are employed, and iv.) the definition of a universal, dynamic,
fault-tolerant, and load-balanced computational framework adapted to all kinds
of computational platforms (massively parallel machines, clusters, or
distributed grids). These strategies have been implemented in the QMC=Chem code
developed at Toulouse and illustrated with numerical applications on small
peptides of increasing sizes (158, 434, 1056 and 1731 electrons). Using 10k-80k
computing cores of the Curie machine (GENCI-TGCC-CEA, France) QMC=Chem has been
shown to be capable of running at the petascale level, thus demonstrating that
for this machine a large part of the peak performance can be achieved.
Implementation of large-scale QMC simulations for future exascale platforms
with a comparable level of efficiency is expected to be feasible
Cluster vs Single-Spin Algorithms -- Which are More Efficient?
A comparison between single-cluster and single-spin algorithms is made for
the Ising model in 2 and 3 dimensions. We compare the amount of computer time
needed to achieve a given level of statistical accuracy, rather than the speed
in terms of site updates per second or the dynamical critical exponents. Our
main result is that the cluster algorithms become more efficient when the
system size, , exceeds, -- for and --
for . The exact value of the crossover is dependent upon the computer
being used. The lower end of the crossover range is typical of workstations
while the higher end is typical of vector computers. Hence, even for
workstations, the system sizes needed for efficient use of the cluster
algorithm is relatively large.Comment: 13pages, postscript file, HLRZ 21/9
Parallelization of the exact diagonalization of the t-t'-Hubbard model
We present a new parallel algorithm for the exact diagonalization of the
-Hubbard model with the Lanczos-method. By invoking a new scheme of
labeling the states we were able to obtain a speedup of up to four on 16 nodes
of an IBM SP2 for the calculation of the ground state energy and an almost
linear speedup for the calculation of the correlation functions. Using this
algorithm we performed an extensive study of the influence of the next-nearest
hopping parameter in the -Hubbard model on ground state energy and
the superconducting correlation functions for both attractive and repulsive
interaction.Comment: 18 Pages, 1 table, 8 figures, Latex uses revtex, submitted to Comp.
Phys. Com
The flat phase of fixed-connectivity membranes
The statistical mechanics of flexible two-dimensional surfaces (membranes)
appears in a wide variety of physical settings. In this talk we discuss the
simplest case of fixed-connectivity surfaces. We first review the current
theoretical understanding of the remarkable flat phase of such membranes. We
then summarize the results of a recent large scale Monte Carlo simulation of
the simplest conceivable discrete realization of this system \cite{BCFTA}. We
verify the existence of long-range order, determine the associated critical
exponents of the flat phase and compare the results to the predictions of
various theoretical models.Comment: 7 pages, 5 figures, 3 tables. LaTeX w/epscrc2.sty, combined
contribution of M. Falcioni and M. Bowick to LATTICE96(gravity), to appear in
Nucl. Phys. B (proc. suppl.
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