Phase transformation and subdomain structure in ͓0001͔-oriented gallium nitride ͑GaN͒ nanorods of different sizes are studied using molecular dynamics simulations. The analysis concerns the structure of GaN nanorods at 300 K without external loading. Calculations show that a transformation from wurtzite to a tetragonal structure occurs along ͕0110͖ lateral surfaces, leading to the formation of a six-sided columnar inversion domain boundary ͑IDB͒ in the ͓0001͔ direction of the nanorods. This structural configuration is similar to the IDB structure observed experimentally in GaN epitaxial layers. The transformation is significantly dependent on the size of the nanorods. © 2009 American Institute of Physics. ͓doi:10.1063/1.3268467͔With a wide band gap, gallium nitride ͑GaN͒ promises superior performances for blue light-emitting diodes, 1 highpower sources, and optical data storage devices.2 Recent advances in the synthesis of one-dimensional nanostructures have brought forth possibilities for GaN to be used in more applications.3 Structural transformations and defects are subjects of active experimental and theoretical research for GaN, 4 because they lead to significant changes in physical and chemical properties and, therefore, affect applications. These transformations and defects can be caused by external or internal factors. For epitaxially grown GaN layers, because of the lack of a lattice and thermally matching substrate, they always contain large numbers of defects.5 Highresolution x-ray diffraction studies of GaN layers have indicated that both carrier mobility and the intensity of photoluminescence are strongly related to defects.6 It was also observed that a phase transformation from wurtzite ͑WZ͒ to rock-salt structure can be induced by high pressure in GaN. 7 However for GaN nanostructures, surface stresses due to high surface-to-volume ratios are another factor that may lead to structural transitions. Experimental observations and atomistic simulations show that structural reorientation can occur in metal ͑copper, silver, and gold͒ nanowires due to high surface-stresses. 8,9 This reorientation causes some of the face-centered-cubic metal nanowires to exhibit a shape memory and pseudoelasticity which are not possessed by corresponding bulk materials. 9 The microscopic details of structural transformations in nanostructures are, however, difficult to investigate through direct experimental observations. Atomistic simulations can play an important role in this regard. Recently, molecular dynamics ͑MD͒ simulations are carried out to analyze the atomistic structures of singlecrystal GaN nanotubes and the buckling behavior of GaN nanowires.10 Yet, the effect of surface-stresses on the structural transformation in GaN nanorods is still poorly understood and merit careful inspection. Here, MD simulations are carried out to investigate the surface transformation and defect formation in GaN nanorods. The analyses concern the equilibrium structure of GaN nanorods with lateral dimensions between 22.6-67.4 Å ...