according to recursion formulas given by Mulliken. Thus, the four parameters 𝑈 𝑟 , 𝛾 𝑟𝑟 , 𝛾 𝑟𝑠 and 𝛽 𝑟𝑠 are expressed completely in terms of the corresponding valence state ionization potentials IP 𝑣 and electron affinities EA 𝑣 as well as the completely empirical parameter 𝐶. We have used the set of valence state ionization potentials and electron affinities from Beveridge and Hinze (Table ). The distance dependence of 𝛽 𝑟𝑠 and 𝛾 𝑟𝑠 allows for treatment of bond-length alternation beyond idealized geometries. In addition, we have followed the literature and added an angle-dependence to 𝛽 𝑟𝑠 according to