Constrained density functional theory calculations for estimation of forward and backward intermolecular charge transfer energy

Charge transfer (CT) in donor-acceptor complexes can occur in two directions: from donor to acceptor (forward CT, CTF) and from acceptor to donor (backward CT, CTB). For an ethylene and tetrafluoroethylene model system, coupled cluster calculations predict that CTF is more stable than CTB. Meanwhile, density functional theory (DFT) and time-dependent DFT (TDDFT) employing B3LYP functional show a different CT state order depending on the Hartree-Fock exchange (HFX) contribution. The impact of basis sets as a possible cause to induce such unreasonable CT state reverse along with the increase of HFX was investigated. The physical origin of CT state reversal is examined in terms of molecular orbital energy. We analyzed two additional molecular pairs that feature CT state reversal along with the amount of HFX in B3LYP functional. Despite the reduced gap, we suggest constrained DFT using M06-HF functional as an effective approach to estimate two CT state energies on equal footing.