Editing Atomic units (section)

== Motivation ==
In the context of atomic physics, using the atomic units system can be a convenient shortcut, eliminating symbols and numbers and reducing the order of magnitude of most numbers involved.
For example, the [[Hamiltonian operator]] in the [[Schrödinger equation]] for the [[helium]] atom with standard quantities, such as when using SI units, is<ref name="McQuarrie2008">{{cite book |last1=McQuarrie |first1=Donald A. |title=Quantum Chemistry |date=2008 |publisher=University Science Books |location=New York, NY |edition=2nd }}</ref>{{rp|[https://books.google.com/books?hl=en&id=zzxLTIljQB4C&pg=PA437 437]}} 
: <math>\hat{H} = - \frac{\hbar^2}{2m_\text{e}} \nabla_1^2  - \frac{\hbar^2}{2m_\text{e}} \nabla_2^2 - \frac{2e^2}{4\pi\epsilon_0 r_1} - \frac{2e^2}{4\pi\epsilon_0 r_2} + \frac{e^2}{4\pi\epsilon_0 r_{12}} ,</math>
but adopting the convention associated with atomic units that transforms quantities into [[Dimensionless quantity|dimensionless]] equivalents, it becomes
: <math>\hat{H} = - \frac{1}{2} \nabla_1^2 - \frac{1}{2} \nabla_2^2  - \frac{2}{r_1}  - \frac{2}{r_2} + \frac{1}{r_{12}} .</math>
In this convention, the constants {{tmath|1= \hbar }}, {{tmath|1= m_\text{e} }}, {{tmath|1= 4 \pi \epsilon_0 }}, and {{tmath|1= e }} all correspond to the value {{tmath|1= 1 }} (see ''{{slink|#Definition}}'' below). 
The distances relevant to the physics expressed in SI units are naturally on the order of {{tmath|1= 10^{-10}\,\mathrm{m} }}, while expressed in atomic units distances are on the order of {{tmath|1= 1 a_0 }} (one [[Bohr radius]], the atomic unit of length). An additional benefit of expressing quantities using atomic units is that their values calculated and reported in atomic units do not change when values of fundamental constants are revised, since the fundamental constants are built into the conversion factors between atomic units and SI.