where projecting the molecular orbitals of the dimer on the basis of the molecular orbitals of the individual molecules. This approach has been demonstrated very effective to calculate the electronic coupling in a variety of situations. However, the Hamiltonian in the new orbital basis lacks the orthonormalization condition and thus a Löwdin's orthonormal transformation is needed. Therefore, the explicit expression for t ab , 44,45 also referred here as Charge Transfer Integral (CTI), is defined in terms of the non-orthonormal Hamiltonian as: