Geometrical models of neutral single vacancy-arsenic complexes are determined from first principles and used for atomistic simulation of Rutherford backscattering channeling (RBS-C) spectra in heavily As-doped crystalline silicon, both with and without compensating Si self-interstitials. The goal is to investigate whether the relaxation patterns of complexes containing different numbers (from 1 to 4) of As atoms can be used as a fingerprint in structural analysis by conventional RBS-C. Simulation of RBS-C spectra in million-atoms supercells containing a population of As m V, show the off-lattice displacement of the Si atoms neighboring the vacancy, due to Jahn-Teller effect. On the other side, As displacement is found to be similar in all clusters investigated. The present results suggest that in the case of samples equilibrated at high temperature, the lack of any significant disorder of Si atoms is consistent with the …