Editing Molecule (section)

== Theoretical aspects ==

The study of molecules by molecular physics and [[theoretical chemistry]] is largely based on [[quantum mechanic]]s and is essential for the understanding of the chemical bond. The simplest of molecules is the [[hydrogen molecule-ion]], H<sub>2</sub><sup>+</sup>, and the simplest of all the chemical bonds is the [[one-electron bond]]. H<sub>2</sub><sup>+</sup> is composed of two positively charged [[proton]]s and one negatively charged [[electron]], which means that the [[Schrödinger equation]] for the system can be solved more easily due to the lack of electron–electron repulsion. With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of [[computational chemistry]].

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 [[helium]] [[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.
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