  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.
