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We describe a new hybrid N-body/hydrodynamical code based on theparticle-mesh (PM) method and the piecewise-parabolic method (PPM) for use insolving problems related to the evolution of large-scale structure, galaxyclusters, and individual galaxies. The code, named COSMOS, possesses severalnew features which distinguish it from other PM-PPM codes. In particular, tosolve the Poisson equation we have written a new multigrid solver which candetermine the gravitational potential of isolated matter distributions andwhich properly takes into account the finite-volume discretization required byPPM. All components of the code are constructed to work with a nonuniform mesh,preserving second-order spatial differences. The PPM code uses vacuum boundaryconditions for isolated problems, preventing inflows when appropriate. The PMcode uses a second-order variable-timestep time integration scheme. Radiativecooling and cosmological expansion terms are included. COSMOS has beenimplemented for parallel computers using the Parallel Virtual Machine (PVM)library, and it features a modular design which simplifies the addition of newphysics and the configuration of the code for different types of problems. Wediscuss the equations solved by COSMOS and describe the algorithms used, withemphasis on these features. We also discuss the results of tests we haveperformed to establish that COSMOS works and to determine its range ofvalidity.Comment: 43 pages, 14 figures, submitted to ApJS and revised according to  referee's comment

COSMOS: A HybridN‐Body/Hydrodynamics Code for Cosmological Problems