Based on the available experimental and computational capabilities, a phenomenological approach has been proposed to formulate a hypersurface in both spatial and temporal domains to predict combined specimen size and loading rate effects on the material properties [1-2]. A systematic investigation is being performed to understand the combined size, rate and thermal effects on the properties and deformation patterns of representative materials with different nanostructures and under various types of loading conditions [3- 16]. The recent study on the single crystal copper response to impact loading has revealed the size-dependence of the Hugoniot curve. In this paper, the “inverse Hall-Petch” behavior as observed in the impact response of single crystal copper, which has not been reported in the open literature, is investigated by performing molecular dynamics simulations of the response of copper nanobeam targets subjected to impacts by copper nanobeam flyers with different impact velocities. It appears from the preliminary results that the “inverse Hall-Petch” behavior in single crystal copper is mainly due to the formation and evolution of disordered atoms and the interaction between ordered and disordered atoms, as compared with the physics behind the “inverse Hall-Petch”behavior as observed in nanocrystalline materials. Based on the available experimental and computational capabilities, a phenomenological approach has been to formulate a hypersurface in both spatial and temporal domains to manage integrated specimen size and loading rate effects on the material properties [1-2]. A systematic investigation is being conducted to understand the combined size, rate and thermal effects on the properties and deformation patterns of representative materials with different nanostructures and under various types of loading conditions [3- 16]. The recent study on the single crystal copper response to impact loading has revealed the size-dependence of the Hugoniot curve. In this paper, the “inverse Hall-Petch” behavior as observed in the impact response of single crystal copper, which has not been reported in the open literature, is investigated by performed molecular dynamics simulations of the response of copper nanobeam targets subjected to impacts by copper nanobeam flyers with different impact ve locities. It appears from the preliminary results that the “inverse Hall-Petch ” behavior in single crystal copper is mainly due to the formation and evolution of disordered atoms and the interaction between ordered and disordered atoms, as compared with the physics behind the “inverse Hall-Petch ” behavior as observed in nanocrystalline materials.