By performing atomistic calculations with a tight-binding potential, we study the displacement field induced by a screw dislocation lying along a free 011 Cu cylindrical nanowire. For this anisotropic orientation that is often encountered experimentally, we show that the displacement field u z along the nanowire can be seen as the superposition of three different fields: the screw dislocation field in an infinite medium, the warping displacement field caused by the so-called Eshelby twist, and an additional image field induced by the free surfaces. A Fourier series analysis of this latter image displacement and stress fields is given. For a circular cross section of the wire, this image field corresponds mainly to an additional warping displacement u z ∝ xy. The dissociation mechanism of the dislocation into partials and the surface stress effects being also captured in our simulations, the present study enables one to quantify the various contributions to the formation of the x-ray diffractograms.