The calculations clearly reveal the origin of two transitions responsible for the spectral features that are common to all four studied complexes. The transitions are illustrated in Figure . It can be seen that the experimental band near 650 nm (Figure ) arises from LP(X) > (M-X)* a1 (a' in the Cs symmetry, vide infra) excitation (Figure ) whereas the experimental band near 500 nm (Figure ) arises from the (M-X) > (M-X)* b2 (a'' in Cs) excitation (Figure ).