Time-dependent density functional theory
Spin-assignment problem in TD-DFT
Perturbative preselection of doubles in dressed TD-DFT
(a) Spin-assignment problem in TD-DFT
Excited states are often treated within the context of time-dependent (TD) density-functional theory (DFT), making it important to be able to assign the excited spin-state symmetry. While there is universal agreement on how ∆ ⟨Sˆ2⟩, the difference between ⟨Sˆ2⟩ for ground and excited states, should be calculated in a wave-function-like formalism such as the Tamm-Dancoff approximation (TDA), confusion persists as to how to determine the spin-state symmetry of excited states in TD-DFT. In this work, we addressed this issue by exploring variations on the formula for calculating ∆⟨S2⟩ in TD-DFT beginning with the one first derived directly from response theory and programmed in deMon2k along with a variation programmed in Gaus- sian09. I also took a close look at the minimum basis broken symmetry description of H2 at different bond lengths and show that the results of the different philosophical choices used for calculating ∆⟨S2⟩.
Hemanadhan Myneni and Mark E. Casida, "On the Calculation of Δ < S2 > for Electronic Excitations in Time-Dependent Density-Functional Theory", Comput. Phys. Comm. 213, 72 (2017). [Link]
(b) Perturbative preselection of doubles in dressed TD-DFT
Treating double excitations within TD-DFT is one of the most challenging problems in quantum chemistry. Dressed TD-DFT addresses this issue by introducing frequency-dependent terms absent in the conventional adiabatic exchange-correlation kernel. In practice, it involves preselecting the dominant 2h2p (double) excitations by adding them to the initial guess of the block Davidson diagonalizer. In this work, we proposed a perturbative preselection procedure to select the dominant doubles.
M. Hemanadhan and Mark E. Casida, "Perturbative Preselection of Doubles in Dressed TD-DFT", at 16"th International Conference on Density Functional Theory and its Applications (DFT-2015), University of Debrecen, Debrecen − Hungary (31 Aug.–04 Sep. 2015).
# Last update: 21 August 2024