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.