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Manuscript Title: CLUSTEREASY A Program for Lattice Simulations of Scalar Fields in an Expanding Universe on Parallel Computing Clusters.
Authors: Gary Felder
Program title: CLUSTEREASY
Catalogue identifier: AEBJ_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 179(2008)604
Programming language: C++/MPI.
Computer: Cluster. Must have the library FFTW installed.
Operating system: Any.
RAM: Typically 4 MB to 1 GB per processor
Keywords: Inflation, Reheating, Lattice Simulations, Classical Field Theory, Parallel Programming.
PACS: 98.80.Jk, 98.80.Bp.
Classification: 1.9.

External routines: A single-precision version of the FFTW library (http://www.fftw.org/ ) must be available on the target machine.

Nature of problem:
After inflation the universe consisted of interacting fields in a high energy, nonthermal state. [1] The evolution of these fields can not be described with standard approximation techniques such as linearization, kinetic theory, or Hartree expansion, and must thus be simulated numerically. Fortunately, the fields rapidly acquire large occupation numbers over a range of frequencies, so their evolution can be accurately modeled with classical field theory. [2] The specific fields and interactions relevant at these high energies are not known, so different models must be tested phenomenologically. In many cases, e.g. those involving symmetry breaking, the wide range of physical time and length scales in the problem requires parallel computing.

Solution method:
CLUSTEREASY solves the equations of motion for interacting scalar fields in an expanding universe. The user describes a particular theory by entering the field potential and its derivatives in a model file and the program then uses a staggered leapfrog method to evolve the field equations and Friedmann equation for the fields and the expansion of the universe. Different processors compute the evolution on subgrids defined by block decomposition, and the processors exchange edge data after each time step to allow for calculation of spatial derivatives.

Restrictions:
In its current form CLUSTEREASY only includes scalar fields and does not include metric perturbations. For 2D and 3D simulations the cluster must already have the (free) libraries FFTW installed.

Additional comments:
CLUSTEREASY is the parallel form of the program LATTICEEASY (AEAW_v1_0), Comp. Phys. Comm 178(2008)929.

Note: The default installation type for FFTW is double-precision so care must be taken to specify single-precision when running the "configure" file associated with the FFTW software package installation.

Running time:
The running time can range from minutes to weeks.

References:
[1] A. D. Linde, Particle Physics and Inflationary Cosmology (Harwood, Chur, Switzerland, 1990).
[2] S. Khlebnikov and I. Tkachev, Phys. Rev. Lett. 77, 219 (1996), (HEP-PH 9603378).