[en] Radiation damage in materials produced by ion beam irradiation has been studied extensively by Monte Carlo methods as well as molecular dynamics (MD) simulations. Monte Carlo codes based on binary collision approximation are used to follow collision cascades induced by fast (keV to MeV) ions and to obtain the initial defect structure. MD models treat defect generation by low energy beams and are mainly focused on the subsequent defect evolution. The present work addresses a few questions regarding the formation and stability of point defects in crystalline Si. Using MD simulations of 1 to 5 keV recoils in Si, we examine the effect of spatial and temporal overlap of single cascades on the defect concentration and its further development by defect diffusion, trapping and annihilation. The interactions between atoms are treated with Stillinger-Weber and Tersoff three body potential. The effect of the surface on the damage accumulation is investigated by assuming both an infinite and a semi-infinite crystal. A perfect initial state of the crystal is considered as well as the case where the irradiation penetrates previously damaged regions. The structure of such regions is obtained by Monte Carlo calculations using the code MARLOWE. This structure serves as an initial crystal state for the MD simulations. Calculations are performed at different temperatures. We discuss how the results relate to experimental observation of a 'reverse' temperature and dose rate dependence for vacancy type defects, i.e. the concentration increases with increasing the implantation temperature and decreasing the dose rate. (author)