Gravitational collisions in cosmological N-body codes

We study the accumulation of errors in cosmological N-body algorithms that are caused by representing the continuous distribution of matter by massive particles, comparing the PPPM and Adaptive Multigrid codes. We use for this a new measure of two-body relaxation suitable for nonstationary gravitating systems. This measure is based on accumulated deflection angles of particle orbits and it does not saturate during evolution. We have found that the role of gravitational collisions in standard PPPM-models is rather high, and in order to avoid collision effects we recommend to use large comoving softening parameters epsilon>=1.0 (in grid units).

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