Bond length alternation along the polyene chain of the PSB 11-cis-retinal chromophore of Rh. Comparison of (a) the MM-fitted values in the X-ray structures (pdb codes: 1U19,29 1L9H,36 1F88,37 and 1HZX;38 chain A), an NMR structure with pdb code of 1JFP,39 and the high resolution double-quantum solid-state NMR data62 for the C10–C15 moiety including vibrational correction that have confidence level of ± 0.025 Å, (b) the results of present ONIOM calculations, and (c) NMR experiments and DFT-EE results.