We describe a block-localized excitation (BLE) method to carry out constrained optimization of block-localized orbitals for constructing valence bond-like, diabatic excited configurations using multistate density functional theory (MSDFT). The method is an extension of the previous block-localized wave function method through a fragment-based ΔSCF approach to optimize excited determinants within a molecular complex. In BLE, both the number of electrons and the electronic spin of different fragments in a whole system can be constrained, whereas electrostatic, exchange, and polarization interactions among different blocks can be fully taken into account of. To avoid optimization collapse to unwanted states, a ΔSCF projection scheme and a maximum overlap of wave function approach have been presented. The method is illustrated by the excimer complex of two naphthalene molecules. With a minimum of eight spin-adapted configurational state functions, it was found that the inversion of La- and Lb- states near the optimal structure of the excimer complex is correctly produced, which is in quantitative agreement with DMRG-CASPT2 calculations and experiments. Trends in the computed transfer integrals associated with excited-state energy transfer both in the singlet and triplet states are discussed. The results suggest that MSDFT may be used as an efficient approach to treat intermolecular interactions in excited states with a minimal active space (MAS) for interpretation of the results and for dynamic simulations, although the selection of a small active space is often system dependent.
Bibliographical noteFunding Information:
This work was partially supported by NSFC (grant nos 21673246 and 21933011) and the Beijing Municipal Science and Technology Commission (grant no. Z191100007219009) at the CAS, and Shenzhen Municipal Science and Technology Innovation Commission (KQTD2017-0330155106581) and the National Natural Science Foundation of China (grant number 21533003) at SZBL. The study was completed for the excimer complex at Minnesota, which was partially supported by the National Institutes of Health (grant number GM046736).
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