As for the computed intensities of the two peaks, the computed value of the oscillator strength for the a1 excitation (f ≈ 1-4•10 -4 , Figure , Table ) at the C2v minima is rather low. We assumed that it was caused by neglecting of the vibronic coupling. To further elaborate on this assumption, the in-plane bending mode (Figure ) of the halogen atom in the [(bipy)MX] + complex is associated with a very low vibrational frequency of a few tens of cm -1 . In energy terms, this corresponds to a computed deformation energy of 2 -4 kJ.mol -1 (vide infra). Interestingly, in the case of [(bipy)AuCl] + complex, the equilibrium geometry at the PBE0/de2TZVPP/Au(PP) level corresponds to the Cs minimum (Figure ); whereas the C2v structure lies ~ 2 kJ.mol -1 above (Figure ). In fact, the latter (C2v) is a transition state with an imaginary mode of i30 cm -1 , whereas the lowest vibrational mode in the Cs minimum (30 cm -1 ) corresponds to soft in-plane chlorine bending. More importantly, for the Cs minimum of [(bipy)AuCl] + the computations predict ~ 100 times higher oscillator strength for the calc(a') excitation in comparison with the calc(a1) in the C2v geometry (f=0.0103 vs 0.0001, c.f. Table ). The predicted wavelength of the calc(a') transition raises from 639 to 665 nm (1.94 to 1.86 eV), which is admittedly somewhat away from the experimental value of 637 nm (1.95 eV), though still acceptable value.