According to Born (1940) and Hill (1977), a material becomes intrinsically unstable when its elastic moduli tensor (or modified one) loses its positive definiteness. It is true for bulk materials, but it is not exactly true in the case of nanowires, due to the so-called surface effect. The surface effect increases as the thickness of the nanowire decreases, and thus the critical strain differs more from the bulk value as the nanowire becomes narrower. MD is useful to verify the basic continuum theory for bulk materials as well as to measure the difference of nanosystems from their bulk counterparts.