  Allochromatium (Alc.) vinosum has a double-peak structure of its absorption
band around 800~nm. Previously, the excitonic origin of this feature has been
demonstrated experimentally, but a detailed understanding still lacks a model
Hamiltonian being able to reproduce absorption as well as exciton relaxation
time scales. Here, we propose a system-bath model which accommodates these
observables. It combines Frenkel exciton theory for a dimerized and
energetically heterogeneous B800 pigment pool with a quantum master equation
approach describing phase and energy relaxation according to an experimental
spectral density. The analysis of this model shows that the LH2 of Alc. vinosum
features an interesting interplay of two excitonic bands, which are originating
from the different pigment pools.
