We investigate torsion about the C-11-C-12 bond mid-point for the S-1 state of 11-cis retinal, using a QTAIM and stress tensor analysis. The QTAIM and stress tensor responses to a torsion +/-alpha increase at a faster rate for the preferred direction of torsion though the CI seam. A QTAIM and stress tensor vector-based analysis provides an alternative way of characterising the asymmetry of the S-1 potential energy surface. In the vicinity of the CI seam the ellipticity epsilon attained minimum values. The application of this analysis to molecular rotary motors is briefly discussed. (C) 2016 Elsevier B.V. All rights reserved.