AbstractNoncovalent interactions are of utmost importance. However, their accurate treatment is still difficult. This is partially induced by the coexistence of many types of interactions and physical phenomena, which hampers generality in simple treatments. The NCI index has been successfully used for nearly over 10 years in order to identify, analyze, and understand noncovalent interactions in a wide variety of systems, ranging from proteins to molecular crystals. In this work, the development and implications of the method will be reviewed, and modern implementations will be presented. Afterward, some sophisticated examples will be given that showcase the current advances toward the fast, robust, and intuitive identification of noncovalent interactions in real space.This article is categorized under:Software > Molecular ModelingQuantum Computing > Theory DevelopmentStructure and Mechanism > Computational Biochemistry and Biophysics