International Journal of Impact Engineering 33 (2006) 285–293 Large-scale molecular dynamics simulations of hyperthermal cluster impact T.C. Germann à Applied Physics Division (X-1-SMMP), Los Alamos National Laboratory, Los Alamos, NM 87545, USA Available online 15 November 2006 Abstract Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times. r 2006 Elsevier Ltd. All rights reserved. Keywords: Molecular dynamics simulations; Copper; Impact; Fragmentation 1. Introduction Non-equilibrium molecular dynamics (MD) simulations have been increasingly used in recent years to provide detailed atomistic insight into a number of dynamic materials phenomena, including shock-induced plasticity [1,2] and phase transitions [3] in metals, fracture [4,5], laser ablation [6,7], and ion-induced sputtering [8,9], just to name a few. Since the only necessary model input into such simulations is the microscopic interatomic force law, the resulting macroscopic behavior can (and often does!) differ from pre-existing expectations, while providing often unique perspectives on familiar phenomena. For instance, we have recently demonstrated the applicability of MD simulations to probe fluid instabilities, in particular the Rayleigh–Taylor instability when a heavy fluid is placed on top of a lighter one in the presence of gravity [10]. Everything from the equation of state, surface tension, viscosity, and so forth, arises naturally from the interatomic force law (a simple pairwise Lennard-Jones potential in that case), providing a challenging microscopic benchmark for continuum-scale models. Dating back to pioneering work by Vineyard and colleagues [11] at Brookhaven, a particularly fruitful application of MD simulations has been to the investigation of collision cascade events, arising from either radiation damage or high-velocity ion implantation. For especially energetic ion impacts, atoms and small clusters of atoms are ejected from the surface, in a process known as ion-induced sputtering. Such experiments ARTICLE IN PRESS www.elsevier.com/locate/ijimpeng 0734-743X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2006.09.049 à Tel.: +1 505 665 9772; fax: +1 505 665 2879. E-mail address: tcg@lanl.gov.