Delta self-consistent-field methods are widely used in studies of electronically excited states. However, the nonaufbau determinants are generally spin-contaminated. Here, we describe a general approach for spin-coupling interactions of open-shell molecules, making use of multistate density functional theory (MSDFT). In particular, the effective exchange integrals that determine spin coupling are obtained by enforcing the multiplet degeneracy of the S+1 state in the MS = S manifold. Consequently, they are consistent with the energy of the high-spin state that is adequately treated by Kohn-Sham density functional theory (DFT) and, thereby, free of double counting of correlation. The method was applied to core excitations of open-shell molecules and compared with those by spin-adapted time-dependent DFT. An excellent agreement with experiment was found employing the BLYP functional and aug-cc-pCVQZ basis set. Overall, MSDFT provides an effective combination of the strengths of DFT and wave function theory to achieve efficiency and accuracy.
Bibliographical noteFunding Information:
This work was supported in part by grants from the Ministry of Science and Technology (Grant No. 2017YFB0203400), Shenzhen Municipal Science and Technology Innovation Commission (KQTD2017-0330155106581), and the National Natural Science Foundation of China (Grant No. 21533003). A.G. was supported by a National Postdoctoral Fellowship of China while the work was initiated.
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