The atomic and electronic structures of the Si(001)-c(4 x 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 x 4) unit cell. The calculated STM images show a close resemblance to the experimental ones. (c) 2005 Elsevier B.V. All rights reserved.