For example, we subtract the reference bond-order for a C-C single bond (0.7874 for the central bond of 1, 3-butadiene twisted 90 • ) for each formal C-C single bond and the reference bond-order for a C=C double bond (1.1958 for the central bond of cis-2-butene) for each formal C=C double bond. This is identical to the definition used with simple Hückel theory but with reference π-bond-orders of 0 and 1, respectively.