La studo de molekuloj pere de la molekula [[fiziko]] kaj la [[teoria kemio]] isestas largelybazita basedĉefe onsur [[quantumkvantuma mechanicmekaniko|kvantumaj mekanikoj]]sandkaj istemas essentialesence forpri thela understandingkompreno ofde thela [[chemicalkemia bondligo]]. TheLa simplestplej ofsimpla moleculesel isla themolekuloj estas la hidrogenmolekula [[hydrogen molecule-ionjono]], H<sub>2</sub><sup>+</sup>, andkaj thela simplestplej ofsimpla allel theĉiuj chemicalkemiaj bondsligoj isestas thela [[oneKovalenta ligo#Unu- kaj tri-elektronaj ligoj|unu-electronelektrona bondligo]]. H<sub>2</sub><sup>+</sup> isestas composedkombinaĵo ofel twodu positivelypozitive chargedŝarĝitaj [[protonprotono]]sj andkaj oneunu negativelynegative chargedŝarĝita [[electronelektrono]], whichkio meanssignifas, thatke thela [[SchrödingerEkvacio equationde Schrödinger]] for thepor systemla cansistemo bepovas solvedesti moresolvita easilypli duefacile topro thela lackmanko ofde electron–electronelektron–elektrona repulsionopozicio. WithPer thela developmentdisvolvigo ofde fastrapidaj digitalciferecaj computerskomputiloj, approximateproksimumaj solutionssolvoj forpor morepli complicatedkomplikaj moleculesmolekuloj becameebliĝis possiblekaj andili areestas oneunu ofel thela mainĉefaj aspectsaspektoj ofde [[computationalkomputika chemistrykemio]].
When trying to define rigorously whether an arrangement of atoms is ''sufficiently stable'' to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the [[potential energy surface]] that is deep enough to confine at least one vibrational state".<ref name="iupac" /> This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction. In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the [[heliumheliumo]] [[Dimer (chemistry)|dimer]], [[helium dimer|He<sub>2</sub>]], which has one vibrational [[bound state]]<ref>{{cite journal |author=Anderson JB |title=Comment on "An exact quantum Monte Carlo calculation of the helium-helium intermolecular potential" [J. Chem. Phys. 115, 4546 (2001)] |journal=J Chem Phys |volume=120 |issue=20 |pages=9886–7 |date=May 2004 |pmid=15268005 |doi=10.1063/1.1704638 |bibcode= 2004JChPh.120.9886A|doi-access=free }}</ref> and is so loosely bound that it is only likely to be observed at very low temperatures.
Whether or not an arrangement of atoms is ''sufficiently stable'' to be considered a molecule is inherently an operational definition. Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to an [[elementary particle]]); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe.
