Shape similarity of charge-transfer (CT) excitation energy curves in a series of donor-acceptor complexes and its description with a transferable energy of CT orbital

A simple nature of charge-transfer (CT) in the prototype complexes D_p-F₂ (D_p=NH₃, H₂O) manifests itself in a very close shape of their CT excitation energy curves ω_CT(R) along the donor-acceptor separation R. It affords a simple orbital description in terms of the CT orbitals (CTOs) obtained with a transformation of the virtual orbitals of the standard local density approximation (LDA). The transferable energy of the relevant CTO as a function of R closely approximates the common shape of ω_CT(R), while the height of the individual curve is determined with the ionization potential of D_p.