We present a computational study by density functional theory (DFT) of entire silicon nanorods with up to 1648 atoms without any periodicity or symmetry imposed. The nanorods have been selected to have varying aspect ratios and levels of surface passivation with hydrogen. The structures of the nanorods have been optimized using a density functional tight-binding approach, while energies and electronic properties have been computed using linear-scaling DFT with plane-wave accuracy with the ONETEP (Skylaris et al2005 J.Chem.Phys.122 084119) program. The aspect ratio and surface passivation (1 × 1 and 2 × 1 reconstructions) along with the size of the nanorods which leads to quantum confinement along all three dimensions, significantly affect their electronic properties. The structures of the nanorods also show interesting behaviour as, depending on their characteristics, they can in certain areas retain the structure of bulk silicon while in other parts significantly deviate from it.