Arsenic diffusivity in heavily doped n-type silicon has been observed to increase strongly with donor concentration. This behaviour has been related to percolation effect, but other explanations, such as mobile As2V clusters (or, more generally, mobile AsDV clusters, with D being a donor), have also been postulated. In this work, we report the modelling and simulation of arsenic diffusion for high donor concentrations based on AsDV mobile clusters, within the framework of the atomistic non-lattice kinetic Monte Carlo model. Expressions for arsenic diffusivity in terms of microscopic parameters have been developed, and the parameter set have been calibrated from basic experiments and ab initio calculations. For donor concentrations above 2 × 1020 cm−3, the model predicts a concentration dependence of arsenic diffusivity with an exponent of 3.5, in close agreement to the experimental observations and quite similar to the percolation model. Long-hop distances of AsDV clusters have been also analysed. The model has been implemented in the atomistic simulator Sentaurus Process KMC. A good agreement with experimental arsenic profiles has been obtained for a wide variety of process conditions, including low ion energy, high dose and amorphizing implants, and annealing temperatures ranging from 750 to 1050 °C. The model has shown to allow efficient and accurate simulation, working together with all the other models accounting for the complex phenomenology of state-of-the-art processes.