The instantaneous interaction energy ΔEint between two fragments A and B in a molecule AB is partitioned into three terms, namely, (1) the quasiclassical electrostatic interaction ΔEelstat between the fragments; (2) the repulsive exchange (Pauli) interaction ΔEPauli between electrons of the two fragments having the same spin, and (3) the orbital (covalent) interaction ΔEoi which comes from polarization and orbital mixing between the fragments. The latter term can be decomposed into contributions of orbitals with different symmetry Γ, which makes it possible to distinguish between σ, π, and δ contributions to bonding (ΔEoi = EΓ). ΔVelstat and ΔEoi are associated with covalent and ionic contributions to the bonding, respectively. Finally, if the density functional used in the calculations contains dispersion corrections (not in our case), then in Eq. ( ) there is another term, Edisp, that takes into account the interactions due to dispersion forces.