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.