Non-covalent interaction (NCI) index:

1. An index revealing both favourable and unfavourable interactions in 3D space.
2. Based on a quantity in density functional theory, the reduced-density-gradient (RDG) via density inhomogeneities due to the formation of weak interactions.
3. This quantity becomes lower as an interaction appears.
4. A plot of RDG vs the density itself identifies and characterises these weak interactions (as peaks at different densities)
5. These low RDG peaks correspond in 3D to weak interactions such as hydrogen bonds or steric clashes.
6. Isosurfaces of the RDG highlight whether the non-covalent interactions (NCIs) are stabilizing or de-stabilizing:
   • A continuous colour coding of the RDG isosurface maps strong and attractive NCIs (such as hydrogen bonds) in blue, van der Waals interactions in green and repulsive interactions (steric clashes) in red.

The NCI index provides an objective and quantitative tool for assessing the reasons for stereoselection