Ground-state projected covalency index of the chemical bond

The criterion of maximum separation of non-degenerated eigenvalues in a density matrix spectrum is utilized to generalize the definition of the standard Wiberg-type bond covalency index on the case of multi-determinant state functions. This novel approach involves factorization of natural-orbital contributions to the ground-state bond-covalency and is essential for the stationarity condition of the electron population analysis within representation of atomic orbitals. The ground-state projecting technique is introduced to facilitate evaluation of bond orders in excited-state molecular systems. The performance of the presented methodology is tested on three small but educational and diversified molecules.