Because the number of slip systems is limited,deformation twinning plays a significant role in the plastic deformation of hexagonal close-packed(hcp)metals and alloys.Recent microcompression experiments point to strong size effects indicating that pyramidal slips can dominate in deformation under compression.In this work,molecular dynamics simulations are applied to investigate the deformation mechanisms of magnesium single crystals at the nanoscale.It is found that pyramidal slip dominates the c-axis compression deformation,which is consistent with experimental observations on microcompression.We also present analysis on the twinnability of an ideal hcp single crystal at the nanoscale.A criterion for deformation twinning is derived by considering the elastic lattice-rotation strain.Under the c-axis tension,the nucleation and growth of a deformed crystal rotated 90° relative to the parent lattice is observed,which is accompanying with both the {10-12} twinning and the basal/prismatic interface growth.