Modeling Molecular J and H Aggregates using Multiple-Davydov D2 Ansatz

Abstract: 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 absorption spectrum of both open and closed chain aggregates over a range of electrostatic nearest neighbour coupling and temperature values, we find the mD2 Ansatz to be necessary for obtaining accurate aggregate absorption spectrum in all parameter regimes considered, while the regular Davydov D2 Ansatz is not sufficient. Establishing relation between the model parameters and the depth of the mD2 Ansatz is the main focus of the 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.