We have studied van der Waals contacts of the carotenoid rhodopin glucoside (RG) with the bacteriochlorophyll pigments absorbing at 800 nm (B800) in the crystal structure of Rhodopseudomonas acidophila, and the hydrogen positions were determined from quantum chemical calculations at the Hartree--Fock (6-31G) level. We have found strong evidence for hydrogen bonding between the B800 BChl and the RG from neighboring protomer units. The binding energy was estimated to be about 2 kcal/mol (700 cm(-1)). CI-singles approach and time-dependent density functional theory calculations of the B800--RG dimer indicate a red-shift (ca 2 nm) of the B800 Q(y) transition, along with a substantial increase of its oscillator strength, probably due to the hydrogen bonding.