Why does the HOMO-LUMO gap calculated using ground-state DFT differ from the HOMO-LUMO gap and excitation energies obtained through TDDFT? What are the fundamental reasons for these discrepancies?
That's normal. Formally, the orbitals in ground state DFT are only optimized to describe one thing and that's the ground state.
While in simple cases the excited state can be approximated by just changing one orbital, that's not actually what happens physically: everything gets reorganized and you get a completely new excited state wavefunction. The further this new excited state wavefunction deviates from the "one orbital change" approximation, the stronger the change of your gaps will be.
Jürgen Weippert Thank you for your response. Do you know under what conditions (molecular/cluster systems) the DFT-based HOMO-LUMO energies and band gap remain consistent with TD-DFT results?
The HOMO-LUMO gap calculated using ground-state DFT represents the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the absence of excited-state effects. In contrast, TDDFT (Time-Dependent DFT) accounts for electronic excitations, including the response of the system to external perturbations, such as light. The discrepancies arise because ground-state DFT does not include electron correlation effects in excited states, leading to an underestimated HOMO-LUMO gap, while TDDFT provides a more accurate description of excited states by incorporating these effects. Additionally, TDDFT considers the dynamic polarization of the system, which is absent in the static ground-state calculations.
Beenish Ajmal
There is no case in which they are identical. That is not a question of consistency, they are just not optimized to be the same.
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