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Peak-shifting in real-time time-dependent density functional theory.

Makenzie R Provorse1, Bradley F Habenicht1, Christine M Isborn1

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Real-time time-dependent density functional theory (RT-TDDFT) can cause unphysical shifts in absorption peaks. This study explains these shifts arise from coupled one-electron transitions, depending on molecular electronic structure.

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Area of Science:

  • Computational Chemistry
  • Quantum Mechanics
  • Spectroscopy

Background:

  • Real-time time-dependent density functional theory (RT-TDDFT) is a computationally efficient method for modeling electron dynamics.
  • The adiabatic approximation in RT-TDDFT can lead to unphysical shifts in absorption peaks during electron dynamics.

Purpose of the Study:

  • Investigate the origin of time-dependent resonances observed in RT-TDDFT spectra.
  • Provide a rationale for the direction and magnitude of peak shifts based on molecular electronic structure.

Main Methods:

  • Employed real-time time-dependent density functional theory (RT-TDDFT) with exact exchange and hybrid functionals.
  • Compared RT-TDDFT results with exact wave function methods.
  • Analyzed time-dependent absorption spectra for small molecules (H2, HeH(+), LiH).

Main Results:

  • Adiabatic RT-TDDFT shows energy shifts in absorption peaks that fluctuate with excited state populations.
  • Exact wave function methods show constant peak energies but varying intensities.
  • RT-TDDFT peak shifts oscillate with time-dependent molecular orbital populations.
  • Peak shifts depend on the frequency and intensity of the applied field.

Conclusions:

  • The study provides the first explanation for time-dependent peak shifts in RT-TDDFT spectra.
  • Shifts are attributed to coupled one-electron transitions to higher and lower energy states.
  • The direction of the energy shift (higher or lower) depends on the relative energetics of these transitions.