Hückel pi bond order ($p_{ij}={\sum }_k{f}_k{c}_{ik}{c}_{jk}$)

For real orthonormal site functions, $p_{ij}={\sum }_k{f}_k{c}_{ik}{c}_{jk}$, where $f_k$ is orbital occupation. It measures the off-diagonal pi density matrix; a complete C–C bond also contains a sigma contribution.