Inspecting molecular aggregate quadratic vibronic coupling effects using squeezed coherent states.

Area of Science:

• Quantum mechanics
• Theoretical chemistry
• Spectroscopy

Background:

• Excitation dynamics in molecular complexes are crucial for understanding energy transfer.
• Accurate theoretical models are needed to simulate absorption and fluorescence spectra.
• The time-dependent variational principle (TDVP) is a common method for these simulations.

Purpose of the Study:

• To systematically compare the validity of the squeezed Davydov D2 (sqD2) ansatz against simpler and exact methods.
• To characterize the performance of different quantum mechanical approaches in describing excitation dynamics.
• To investigate the impact of vibrational modes and vibronic coupling on spectral simulations.

Main Methods:

• Utilized the time-dependent variational principle (TDVP) with three ansätze: Davydov D2, squeezed D2 (sqD2), and multiple D2 (mD2).
• Performed numerical simulations of absorption and fluorescence spectra for molecular aggregates.
• Included intra- and intermolecular vibrational modes, and quadratic electronic-vibrational (vibronic) coupling.

Main Results:

• The sqD2 ansatz only matched the exact mD2 spectra in a simplified model without quadratic vibronic coupling.
• Accurate spectral modeling of dimers and larger aggregates generally requires the mD2 ansatz.
• For J dimer aggregates coupled to phonon baths, all three ansätze yielded qualitatively similar spectra.
• Quadratic vibronic coupling significantly impacts spectral lineshapes and introduces temperature-dependent shifts.

Conclusions:

• The squeezed Davydov D2 (sqD2) ansatz does not offer substantial improvements over the basic Davydov D2 approach.
• The numerically exact multiple Davydov D2 (mD2) ansatz is often necessary for accurate spectral simulations of molecular aggregates.
• Understanding vibronic coupling is essential for interpreting spectral features and their temperature dependence.