  Layered superatomic semiconductors, whose buildings blocks are atomically
precise molecular clusters, exhibit interesting electronic and vibrational
properties. In recent work [Science 382, 438 (2023)], transient reflection
microscopy revealed quasi-ballistic exciton dynamics in Re6Se8Cl2, which was
attributed to the formation of polarons due to coupling with acoustic phonons.
Here, we characterize the electronic, excitonic, and phononic properties with
periodic density functional theory. We further parameterize a polaron
Hamiltonian with nonlocal [Su-Schrieffer-Heeger (SSH)] coupling to acoustic
phonon to study the polaron ground state binding energy and dispersion relation
with variational wavefunctions. We calculate a polaron binding energy of about
10 meV at room temperature, and the maximum group velocity of our polaron
dispersion relation is 1.5 km/s, which is similar to the experimentally
observed exciton transport velocity.
