Editing Intermolecular force (section)

==Quantum mechanical theories==

{{Main|Covalent bond#Quantum mechanical description}}

Intermolecular forces observed between atoms and molecules can be described phenomenologically as occurring between permanent and instantaneous dipoles, as outlined above. Alternatively, one may seek a fundamental, unifying theory that is able to explain the various types of interactions such as [[hydrogen bonding]],<ref name=":0">{{Cite journal| vauthors = Arunan E, Desiraju GR, Klein RA, Sadlej J, Scheiner S, Alkorta I, Clary DC, Crabtree RH, Dannenberg JJ, Hobza P, Kjaergaard HG | display-authors = 6 |date=2011-07-08|title=Definition of the hydrogen bond (IUPAC Recommendations 2011)|journal=Pure and Applied Chemistry|volume=83|issue=8|pages=1637–1641|doi=10.1351/PAC-REC-10-01-02|s2cid=97688573|issn=1365-3075|doi-access=free}}</ref> [[van der Waals force]]<ref name=LD>{{cite book | vauthors = Landau LD, Lifshitz EM | title = Electrodynamics of Continuous Media | url = https://archive.org/details/electrodynamicso00land | url-access = registration | publisher = Pergamon | location = Oxford | date = 1960 | pages = [https://archive.org/details/electrodynamicso00land/page/368 368–376] }}</ref> and dipole–dipole interactions. Typically, this is done by applying the ideas of [[quantum mechanics]] to molecules, and Rayleigh–Schrödinger [[perturbation theory]] has been especially effective in this regard. When applied to existing [[quantum chemistry]] methods, such a quantum mechanical explanation of intermolecular interactions provides an array of approximate methods that can be used to analyze intermolecular interactions.<ref>{{cite journal | doi = 10.1021/ja00428a004 | title = Theory of the Chemical Bond | year = 1976 | vauthors = King M | journal = Journal of the American Chemical Society | volume = 98 | issue = 12 | pages = 3415–3420 }}</ref>  One of the most helpful methods to visualize this kind of intermolecular interactions, that we can find in quantum chemistry, is the [[Non-covalent interactions index|non-covalent interaction index]], which is based on the electron density of the system. London dispersion forces play a big role with this.

Concerning electron density topology, recent methods based on electron density gradient methods have emerged recently, notably with the development of IBSI (Intrinsic Bond Strength Index),<ref>{{cite journal | vauthors = Klein J, Khartabil H, Boisson JC, Contreras-García J, Piquemal JP, Hénon E | title = New Way for Probing Bond Strength | journal = The Journal of Physical Chemistry A | volume = 124 | issue = 9 | pages = 1850–1860 | date = March 2020 | pmid = 32039597 | doi = 10.1021/acs.jpca.9b09845 | s2cid = 211070812 | bibcode = 2020JPCA..124.1850K | url = https://hal.archives-ouvertes.fr/hal-02377737/file/4_IGM_bond13012020_11h49%20%281%29.pdf }}</ref> relying on the IGM (Independent Gradient Model) methodology.<ref>{{cite journal | vauthors = Lefebvre C, Rubez G, Khartabil H, Boisson JC, Contreras-García J, Hénon E | title = Accurately extracting the signature of intermolecular interactions present in the NCI plot of the reduced density gradient versus electron density | journal = Physical Chemistry Chemical Physics | volume = 19 | issue = 27 | pages = 17928–17936 | date = July 2017 | pmid = 28664951 | doi = 10.1039/C7CP02110K | bibcode = 2017PCCP...1917928L | url = https://hal.univ-reims.fr/hal-02505160/file/2017_IGM_PROMOL%20%281%29.pdf }}</ref><ref>{{cite journal | vauthors = Lefebvre C, Khartabil H, Boisson JC, Contreras-García J, Piquemal JP, Hénon E | title = The Independent Gradient Model: A New Approach for Probing Strong and Weak Interactions in Molecules from Wave Function Calculations | journal = ChemPhysChem | volume = 19 | issue = 6 | pages = 724–735 | date = March 2018 | pmid = 29250908 | doi = 10.1002/cphc.201701325 | url = https://hal.univ-reims.fr/hal-03377532/file/37_version.pdf }}</ref><ref>{{cite journal | vauthors = Ponce-Vargas M, Lefebvre C, Boisson JC, Hénon E | title = Atomic Decomposition Scheme of Noncovalent Interactions Applied to Host-Guest Assemblies | journal = Journal of Chemical Information and Modeling | volume = 60 | issue = 1 | pages = 268–278 | date = January 2020 | pmid = 31877034 | doi = 10.1021/acs.jcim.9b01016 | s2cid = 209488458 }}</ref>