Abstract <p>In this work, the mechanisms of cascade collapse were investigated using molecular dynamics simulation method for copper. For this purpose, the simulations were carried out using the molecular dynamics code MOLDYCASK. In addition to the density difference between the surrounding crystal and the molten zone, the difference in atomic density, quantified by the initial vacancy concentration, plays a critical role in cascade collapse as it directly influences vacancy transport and atomic diffusion during the thermal spike stage. Vacancy transport during the cascade thermal spike stage is determined by mean square displacement (MSD), the enhancement of mean square displacement due to an introduction of vacancies into the cascade volume. At the melting point of the material, mean square displacement is affected by the coefficient of vacancy diffusion, which is heavily influenced by initial vacancy density rather than deposited energy density.</p>