Absorption and Circular Dichroism Spectra of Molecular Aggregates With the Full Cumulant Expansion

The
exciton Hamiltonian of multichromophoric aggregates can be
probed by spectroscopic techniques such as linear absorption and circular
dichroism. To compare calculated Hamiltonians to experiments, a lineshape
theory is needed, which takes into account the coupling of the excitons
with inter- and intramolecular vibrations. This coupling is normally
introduced in a perturbative way through the cumulant expansion formalism
and further approximated by assuming a Markovian exciton dynamics,
for example with the modified Redfield theory. Here, we present the
implementation of the full cumulant expansion (FCE) formalism (J. Chem. Phys. 142, 2015, 094106) to efficiently
compute absorption and circular dichroism spectra of molecular aggregates
beyond the Markov approximation, without restrictions on the form
of exciton–phonon coupling. By employing the LH2 system of
purple bacteria as a challenging test case, we compare the FCE lineshapes
with the Markovian lineshapes obtained with the modified Redfield
theory, showing that the latter presents a less satisfying agreement
with experiments. The FCE approach instead accurately describes the
lineshapes, especially in the vibronic sideband of the B800 peak.
We envision that the FCE approach will become a valuable tool for
accurately comparing model exciton Hamiltonians with optical spectroscopy
experiments.