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

We present a systematic comparison of three quantum mechanical approaches describing excitation dynamics in molecular complexes using the time-dependent variational principle (TDVP) with increasing sophistication trial wavefunctions (ansatze): Davydov D2, squeezed D2 (sqD2) and a numerically exact multiple D2 (mD2) ansatz in order to characterize validity of the sqD2 ansatz. Numerical simulations of molecular aggregate absorption and fluorescence spectra with intra- and intermolecular vibrational modes, including quadratic electronic-vibrational (vibronic) coupling term, which is due to vibrational frequency shift upon pigment excitation are presented. Simulated absorption and fluorescence spectra of a J type molecular dimer with high frequency intramolecular vibrational modes obtained with D2 and sqD2 ansatze match the spectra of mD2 ansatz only in the single pigment model without quadratic vibronic coupling. In general, the use of mD2 ansatz is required to model an accurate dimer and larger aggregate's spectra. For a J dimer aggregate coupled to a low frequency intermolecular phonon bath, absorption and fluorescence spectra are qualitatively similar using all three ansatze. The quadratic vibronic coupling term in both absorption and fluorescence spectra manifests itself as a lineshape peak amplitude redistribution, static frequency shift and an additional shift, which is temperature dependent. Overall the squeezed D2 model does not result in a considerable improvement of the simulation results compared to the simplest Davydov D2 approach.

Modeling molecular J and H aggregates using multiple-Davydov D2 ansatz.

The linear absorption spectrum of J and H molecular aggregates is studied using the time-dependent Dirac-Frenkel variational principle (TDVP) with the multi-Davydov D2 (mD2) trial wavefunction (Ansatz). Both the electronic and vibrational molecular degrees of freedom (DOF) are considered. By inspecting and comparing the absorption spectrum of both open and closed chain aggregates over a range of electrostatic nearest neighbor coupling and temperature values, we find that the mD2 Ansatz is necessary for obtaining an accurate aggregate absorption spectrum in all parameter regimes considered, while the regular Davydov D2 Ansatz is not sufficient. Establishing a relationship between the model parameters and the depth of the mD2 Ansatz is the main focus of this study. Molecular aggregate wavepacket dynamics, during excitation by an external field, is also studied. We find the wavepacket to exhibit an out-of-phase oscillatory behavior along the coordinate and momentum axes and an overall wavepacket broadening, implying the electron-vibrational (vibronic) eigenstates of an aggregate to reside on non-parabolic energy surfaces.