Noncovalent interactions are ubiquitous in Nature and contribute to the cohesion of chemical systems. If their existence was long suspected they were firstly taken into consideration by van der Waals in 1873, helping to revise the equation of state for real gases. In comparison to covalent bonds, intra- and intermolecular noncovalent interactions are in general locally weak and exhibit much lower energy and directionality, as reflected by the term ''noncovalent''. Nevertheless, in many cases these interactions can collectively play a dominant role in synthesis, catalysis and design of materials. In the first half of the 20th century, these properties have already fueled passionate debates on their origin and nature particularly with the emergence of quantum chemistry with the works of Schrödinger and London.
Based on the nature of the particular elements or synthons involved in the interactions, noncovalent bonds are often classified into hydrogen, noble gases, halogen, chalcogen, pnictogen, tetrel and triel «bonds», as well as agostic, anagostic, cation-π, anion-π, n-π*, π-π stacking, metal-metal, dispersion-driven and hydrophobic interactions, etc.