The intra- and intermolecular bonds in AuCl(CO) are investigated at the Hartree-Fock, density functional and second-order Moller-Plesset levels using a triple-xi -valence-plus-polarization basis with effective core potentials and two additional f-type functions on the gold atom. Calculations on molecular fragments, the AuCl(CO) molecule, and a head-tail [AuCl(CO)](2) dimer are compared with experimental data of vibrational frequencies and interatomic distances in the crystal. Insight is gained on the nature of the bonding in these type of complexes, especially the elusive intermolecular interaction. For the monomer, the extent of a-donation and pi -back-donation in the Au-(CO) interaction is analyzed in detail. For the dimer, the good agreement between the solid-state structure and the simple head-tail model suggests that the crystal geometry is due primarily to electrostatic interactions? though a more elaborate analysis of the charge density reveals also weak covalent Au-Au and C-Cl bonds.