Non-covalent interaction analysis

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. The RDG can be filtered to remove covalent regions,
4. Isosurfaces of the filtered RDG highlight only the non-covalent interactions (NCIs):
   • Colour-coding isosurface using the appropriate eigenvalue (sign(λ₂)ρ) of the density Hessian
   • Maps strong/attractive non-covalent interactions (hydrogen bonds) in blue
   • Maps dispersion stabilising interactions in cyan-2-green
   • Maps destabilising interactions (steric clashes) in yellow-2-red.

The NCI index provides an objective and quantitative tool for assessing the (non-covalent) contributions to stereoselection: J. Contreras-García et al, doi: psk and dcddrv