Cor atom (a) = α a (4b) and the spins of the atoms S a , and includes a fitted parameter K LOT that only depends on the level of theory used. Given this, Melius-type BACs require fitting 3N + 1 parameters for a given level of theory considering N elements. This is usually smaller than the number of parameters in Petersson-type BAC's. An additional advantage of the Melius-type BACs is that they do not require the bonds to be assigned integer bond-orders, so they are better for molecules with more than one plausible Lewis structure such as aromatics.