According to Koopmans theorem, the derivative of the energy of a canonical MO with respect to nuclear coordinates quantifies its bonding/antibonding character. This quantity allows predictions of bond length variation upon ionisation in a panel of 19 diatomic species. In polyatomic molecules, the derivative of a MO energy with respect to a given bond length reveals the nature and the degree of the bonding/antibonding contribution of this MO with respect to this bond. Accordingly, the HOMO “lone pairs” of CO and CN and the HOMO-2 of CH3CN are found to be antibonding with respect to the C-X bond (X = N, O), whereas the HOMO of N2 is found to be bonding. With the same approach, the variation of the bonding character in the MOs of CO and CH3CN upon interaction with an electron acceptor (modelled through the approach of a proton) or by applying an electric field was studied.