We present density functional calculations of the bonding structures and diffusion barriers for a Cl adatom on Si(001)-(2×1). Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and ~1.1 eV higher in energy than the DB is found. The calculated properties of this state agree with recent ESDIAD and HREELS observations. This bridge-bonded site is not along the Cl intradimer diffusion pathway of lowest energy. For this path a transition state also having a bridging structure (but not breaking the dimer bond) and rather low in energy ( ~0.6 eV with respect to the DB) is determined. The low intradimer barrier is consistent with the facile switching of Cl recently observed in scanning tunneling microscopy experiments.