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 D 2 , squeezed D 2 (sqD 2 ) and a numerically exact multiple D 2 (mD 2 ) ansatz in order to characterize validity of the sqD 2 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 D 2 and sqD 2 ansatze match the spectra of mD 2 ansatz only in the single pigment model without quadratic vibronic coupling. In general, the use of mD 2 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 D 2 model does not result in a considerable improvement of the simulation results compared to the simplest Davydov D 2 approach.