Figure 5. Comparison of the interaction energies for selected dimer geometries of a pair of rigid monomers (black lines) and of a rigid plus a stretched (the two C-O bonds of monomer a have been respectively stretched and shrunk of 10% with respect to equilibrium) monomer (red dashed lines). In particular the stretched C-O bond is the one lying on the intermolecular separation R and pointing closer to the monomer b in the L configuration. The Ta configuration refers to that with the stretched monomer a and the unstretched monomer b perpendicular to it and lying on the line connecting the two molecular CM. Bond-bond results are plotted in the lower panel while those corresponding to present ab initio calculations are in the upper panel.