A QTAIM investigation of the torsion in both the clockwise and counter-clockwise directions about the pi bonds of the retinal protonated Schiff base chromophore in the lowest electronically excited state provided a detailed description of the changing bonding topology that was sensitive to both the torsion and the direction of torsion. Analysis of the separate torsion calculations demonstrated a clear order of preferred bonding torsion in terms of principles of maximizing bonding. Unusual behavior of the bond ellipticity in the S-1 state was found for the two less favorable bonds for torsion and explained in terms of the presence of weak bonding interactions. (C) 2017 Elsevier B.V. All rights reserved.