Non-covalent interactions index

The Non-Covalent Interactions index, commonly referred to as simply Non-Covalent Interactions (NCI) is a visualization index based in the electron density (ρ) and the reduced density gradient (s), which is a scalar field of the former. It is based on the empirical observation that non-covalent interactions can be associated with the regions of small reduced density gradient at low electronic densities. In quantum chemistry, the non-covalent interactions index is used to unambiguously visualize non-covalent interactions.