Figure 2 Contour plots of the total electron density (in e/ų) for the dimeric unit of AuCl(CO), as obtained from several theoretical methods: HFs, HFu, DFu, and MP2u (see text for definition). In correspondence with the intermolecular (3,-1) (s1,s2,s3), and (3,+1) (m1,m2) critical points, the eigenvectors of the Hessian of the density are plotted as arrows normalized to an arbitrary length of 0.75 Å. The density hole around each gold nucleus is due to a pseudopotential, that allows to neglect the core electrons in the calculations. Isodensity lines were plotted every 0.025 units, and additionally every 0.00025 units up to 0.015 units to capture the low density in the intermolecular region. The graph is 4.0 × 8.0 Å wide; the radius of the white area containing each atomic label is 1/8 of the radius used for the ESP fit of that atom. The HFs, DFu, and MP2u plots show one Au-Au (s1) and two C-Cl (s2,s3) saddle points, corresponding to chemical bonds, separated by two local minima (m1,m2); however, if the HFs structure is left free to relax to its unconstrained geometrical optimum HFu, the C-Cl saddle points are substituted by Au-Cl saddle points.