New concept beyond Kohn-Sham DFT
Kohn-Sham Density functional theory (KS-DFT) has been tremendously successful in chemistry and physics. Yet, it is unable to describe the energy degeneracy of spin-multiplet components with any approximate functional, ultimately limiting its application to low-spin states in transition metal chemistry, bond dissociation and photochemistry. In a study entitled, "Spin-Multiplet Components and Energy Splittings by Multistate Density Functional Theory," published in the Journal of Physical Chemistry Letters, graduate students Adam Grofe from the University of MInnesota, and Xin Chen of Jilin University, China, describe a multiconfigurational multistate density function theory (MSDFT) to resolve this fundamental problem in Kohn-Sham density functional approximation. Both Grofe and Chen, who visited Minnesota in the spring, are advised by Chemistry Professor Jiali Gao. Professor Wenjian Liu from the Beijing National Laboratory for Molecular Sciences and College of Chemistry and Molecular Engineering, Beijing, China, also collaborated on this research.
Traditionally, KS-DFT uses a single Slater determinant to represent the ground state density. MSDFT employs multiconfigurational theory by introducing a transition density functional to account for electronic coupling, a concept beyond the Kohn-Sham approximation. In addition to representing spin-multiplet components with the correct symmetry and degeneracy, MSDFT provides a convenient procedure to determine multiplet energy gaps, such as singlet-triplet energy splitting of diradicals.
The figure below illustrates the computed adiabatic and vertical excitation energies of carbene in excellent agreement with experiment and previous computations. MSDFT follows a computational ansatz that includes dynamic correlation first into the individual states via KS-DFT, followed by configuration interaction to incorporate static correlation in the second computational step. Consequently, only a minimum of four configurational states is sufficient for estimating S-T energy gaps of diradicals. On the other hand, traditional multiconfigurational self-consistent field methods such as the complete-active-space (CASSCF) approach typically require a much larger number of configurations, which is further exacerbated by the need for inclusion of dynamic correlation correction using the extremely time-demanding post-SCF perturbation theory.MSDFT is a general theoretical approach, allowing electronically localized diabatic states to be constructed in the beginning. Consequently, it can be used to determine the transfer integrals needed to estimate the rates for electron transfer and excited energy transfer as well as for proton-coupled electron transfer reactions. MSDFT also overcomes the short-coming of time-dependent density functional theory (TD-DFT) for treating conical intersection in the excited state. Thus, it can be used to study photochemical processes.
Gao, J.; Grofe, A.; Ren, H.; Bao, P. J. Phys. Chem. Lett., 2016, 7(24), 5143–5149