  We've studied the B800 part of Rhodoblastus acidophilus light-harvesting
complex (LH2) by several quantum chemical techniques based on the density
functional theory (DFT) and determined the specific method and a minimal
reliable model suitable for further studies of the LH2. In addition to
bacteriochlorophyll a molecules, the minimal model includes two $\alpha$ and
one $\beta$ chain amino acids. Within the model, we are able to reproduce the
contribution of the B800 ring of nine bacteriochlorophyll a molecules to the
near infrared $Q_y$ absorption band. We also discuss the use of hybrid DFT
calculations for precise energy and optical estimations and DFT-based tight
binding (DFTB) method for the large-scale calculations. Crucial importance of
Hartree-Fock exchange interaction for the correct description of B800 peak
position was shown.
