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| '''Atomic units''' ('''au''' or '''a.u.''') form a [[systems of measurement|system]] of [[natural units]] which is especially convenient for [[atomic physics]] calculations. There are two different kinds of atomic units, '''[[Hartree]] atomic units'''<ref name="Hartree28">{{Cite news|last=Hartree|first=D. R.|title=The Wave Mechanics of an Atom with a Non-Coulomb Central Field. Part I. Theory and Methods|year=1928|periodical = Mathematical Proceedings of the Cambridge Philosophical Society | publisher = Cambridge University Press | volume = 24 | issue = 1 | pages = 89–110 | url = http://journals.cambridge.org/action/displayAbstract?aid=1733252 | doi = 10.1017/S0305004100011919}}</ref> and '''[[Rydberg constant|Rydberg]] atomic units''', which differ in the choice of the unit of mass and charge. This article deals with '''Hartree atomic units'''. In atomic units, the numerical values of the following four fundamental [[physical constants]] are all unity by definition:
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| * [[Electron mass]] <math>\!m_\mathrm{e}</math>;
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| * [[Elementary charge]] <math>\!e</math>;
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| * Reduced [[Planck's constant]] <math>\hbar = h/(2 \pi)</math>;
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| * [[Coulomb's constant]] <math>1/(4 \pi \epsilon_0)</math>.
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| Atomic units are often abbreviated "a.u." or "au", not to be confused with the same abbreviation used also for [[astronomical unit]]s, [[arbitrary unit]]s, and [[absorbance|absorbance units]] in different contexts.
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| == Use and notation ==
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| Atomic units, like [[SI units]], have a unit of mass, a unit of length, and so on. However, the use and notation is somewhat different from SI.
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| Suppose a particle with a mass of ''m'' has 3.4 times the mass of electron. The value of ''m'' can be written in three ways:
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| * "<math>m = 3.4~m_e</math>". This is the clearest notation (but least common), where the atomic unit is included explicitly as a symbol.<ref name=Pilar>{{cite book|last=Pilar|first=Frank L.|title=Elementary Quantum Chemistry|year=2001|publisher=Dover Publications|isbn=978-0-486-41464-5|page=155|url=http://books.google.com/books?id=XpGM7r69LdkC&pg=PA155}}</ref>
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| * "<math>m = 3.4~\mathrm{a.u.}</math>" ("a.u." means "expressed in atomic units"). This notation is ambiguous: Here, it means that the mass ''m'' is 3.4 times the atomic unit of mass. But if a length ''L'' were 3.4 times the atomic unit of length, the equation would look the same, "<math>L = 3.4~\mathrm{a.u.}</math>" The dimension needs to be inferred from context.<ref name=Pilar />
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| * "<math>m = 3.4</math>". This notation is similar to the previous one, and has the same dimensional ambiguity. It comes from formally setting the atomic units to 1, in this case <math>m_e = 1</math>, so <math>3.4~m_e = 3.4</math>.<ref>{{cite book|last=Bishop|first=David M.|title=Group Theory and Chemistry|year=1993|publisher=Dover Publications|isbn=978-0-486-67355-4|page=217|url=http://books.google.com/books?id=l4zv4dukBT0C&pg=PA217}}</ref><ref>{{cite book|last=Drake|first=Gordon W. F.|title=Springer Handbook of Atomic, Molecular, and Optical Physics|year=2006|edition=2nd|publisher=Springer|isbn=978-0-387-20802-2|page=5|url=http://books.google.com/books?id=Jj-ad_2aNOAC&pg=PA5}}</ref>
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| == Fundamental atomic units ==
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| These four fundamental constants form the basis of the atomic units (see above). Therefore, their numerical values in the atomic units are unity by definition.
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| {|class="wikitable"
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| |+ Fundamental atomic units
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| |-
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| !scope="col"| Dimension
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| !scope="col"| Name
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| !scope="col"| Symbol/Definition
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| !scope="col"| Value in SI units<ref name="NIST">{{Cite news|title=The NIST Reference on Constants, Units and Uncertainty | publisher=National Institute of Standard and Technology | url=http://physics.nist.gov/cuu/index.html | accessdate=1 April 2012}}</ref>
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| |-
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| | [[mass]] || [[electron rest mass]] || <math>\!m_\mathrm{e}</math> || {{val|fmt=commas|9.10938291|(40)|e=-31|u=kg}}
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| |-
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| | [[electric charge|charge]] || [[elementary charge]] || <math>\!e</math> || {{val|fmt=commas|1.602176565|(35)|e=-19|u=C}}
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| |-
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| | [[action (physics)|action]] || [[reduced Planck's constant]] ||<math>\hbar = h/(2 \pi)</math> ||{{val|fmt=commas|1.054571726|(47)|e=-34|u=J·s}}
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| |-
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| | [[electric constant]]<sup>−1</sup> || [[Coulomb force constant]] ||<math>1/(4 \pi \epsilon_0)</math> ||{{val|fmt=commas|8.9875517873681|e=9|u=kg·m<sup>3</sup>·s<sup>-2</sup>·C<sup>-2</sup>}}
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| |}
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| == Related physical constants ==
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| [[Dimensionless physical constant]]s retain their values in any system of units. Of particular importance is the [[fine-structure constant]] <math>\alpha = \frac{e^2}{(4 \pi \epsilon_0)\hbar c} \approx 1/137</math>. This immediately gives the value of the [[speed of light]], expressed in atomic units.
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| {|class="wikitable"
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| |+ Some physical constants expressed in atomic units
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| |-
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| !scope="col"| Name
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| !scope="col"| Symbol/Definition
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| !scope="col"| Value in atomic units
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| |-
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| | [[speed of light]] || <math>\!c</math> || <math>\!1/\alpha \approx 137</math>
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| | [[classical electron radius]] || <math>r_\mathrm{e}=\frac{1}{4\pi\epsilon_0}\frac{e^2}{m_\mathrm{e} c^2}</math> || <math>\!\alpha^2 \approx 5.32\times10^{-5}</math>
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| |-
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| | [[proton mass]] || <math>m_\mathrm{p}</math> || <math>m_\mathrm{p}/m_\mathrm{e} \approx 1836</math>
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| |-
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| |}
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| == Derived atomic units ==
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| Below are given a few derived units. Some of them have proper names and symbols assigned, as indicated in the table. ''k''<sub>B</sub> is [[Boltzmann constant]].
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| {|class="wikitable"
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| |+ Derived atomic units
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| |-
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| !scope="col"| Dimension
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| !scope="col"| Name
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| !scope="col"| Symbol
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| !scope="col"| Expression
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| !scope="col"| Value in SI units
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| !scope="col"| Value in more common units
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| |-
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| | [[length]] || [[Bohr radius|bohr]] || <math>\!a_0</math> || <math>4\pi \epsilon_0 \hbar^2 / (m_\mathrm{e} e^2) = \hbar / (m_\mathrm{e} c \alpha) </math> || {{val|fmt=commas|5.2917721092|(17)|e=-11|u=m}}<ref name="NIST">{{Cite news|title=The NIST Reference on Constants, Units and Uncertainty | publisher=National Institute of Standard and Technology | url=http://physics.nist.gov/cgi-bin/cuu/Value?bohrrada0 | accessdate=21 January 2014}}</ref>|| {{val|fmt=commas|0.052917721092|(17)|u=nm}}={{val|fmt=commas|0.52917721092|(17)|u=Å}}
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| | [[energy]] || [[hartree]] || <math>\!E_\mathrm{h}</math> || <math>m_\mathrm{e} e^4/(4\pi\epsilon_0\hbar)^2 = \alpha^2 m_\mathrm{e} c^2 </math> || {{val|fmt=commas|4.35974417|(75)|e=-18|u=J}} || {{val|fmt=commas|27.211|u=eV}}={{val|fmt=commas|627.509|u=kcal·mol<sup>−1</sup>}}
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| |-
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| | [[time]] || || || <math>\hbar / E_\mathrm{h}</math> || {{val|fmt=commas|2.418884326505|(16)|e=-17|u=s}}||
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| | [[velocity]] || || || <math> a_0 E_\mathrm{h} / \hbar = \alpha c</math> || {{val|fmt=commas|2.1876912633|(73)|e=6|u=m·s<sup>−1</sup>}} ||
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| | [[force]] || || || <math>\! E_\mathrm{h} / a_0 </math> || {{val|fmt=commas|8.2387225|(14)|e=-8|u=N}}||{{val|fmt=commas|82.387|u=nN}}={{val|fmt=commas|51.421|u=eV·Å<sup>−1</sup>}}
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| | [[temperature]] || || || <math>\! E_\mathrm{h} / k_\mathrm{B} </math>|| {{val|fmt=commas|3.1577464|(55)|e=5|u=K}}||
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| | [[pressure]] || || || <math> E_\mathrm{h} / {a_0}^3 </math> || {{val|fmt=commas|2.9421912|(19)|e=13|u=Pa}} ||
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| |[[electric field]] || || || <math>\!E_\mathrm{h} / (ea_0) </math> || {{val|fmt=commas|5.14220652|(11)|e=11|u=V·m<sup>−1</sup>}}|| {{val|fmt=commas|5.14220652|(11)|u=GV·cm<sup>−1</sup>}}={{val|fmt=commas|51.4220652|(11)|u=V·Å<sup>−1</sup>}}
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| |[[electric dipole moment]] || || || <math> e a_0 </math> || {{val|fmt=commas|8.47835326|(19)|e=-30|u= C·m}} || {{val|fmt=commas|2.541746|u=D}}
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| |}
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| == SI and Gaussian-CGS variants, and magnetism-related units ==
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| There are two common variants of atomic units, one where they are used in conjunction with [[SI units]] for [[electromagnetism]], and one where they are used with [[Gaussian units|Gaussian-CGS units]].<ref>{{cite web|title=A note on Units|url=http://www.colorado.edu/physics/phys7550/phys7550_sp07/extras/Appendix_1.pdf|work=Physics 7550 — Atomic and Molecular Spectra|publisher=University of Colorado lecture notes}}</ref> Although the units written above are the same either way (including the unit for electric field), the units related to magnetism are not. In the SI system, the atomic unit for magnetic field is
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| : 1 a.u. = <math>\frac{\hbar}{e a_0^2}</math> = {{val|2.35|e=5}} [[Tesla (unit)|T]] = {{val|2.35|e=9}} [[gauss (unit)|G]],
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| and in the Gaussian-cgs unit system, the atomic unit for magnetic field is
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| : 1 a.u. = <math>\frac{e}{a_0^2} </math> = {{val|1.72|e=3}} [[Tesla (unit)|T]] = {{val|1.72|e=7}} [[gauss (unit)|G]].
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| (These differ by a factor of [[fine structure constant|α]].)
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| Other magnetism-related quantities are also different in the two systems. An important example is the [[Bohr magneton]]: In SI-based atomic units,<ref>{{cite web|last=Chis|first=Vasile|title=Atomic Units; Molecular Hamiltonian; Born-Oppenheimer Approximation|url=http://phys.ubbcluj.ro/~vchis/cursuri/cspm/course2.pdf|work=Molecular Structure and Properties Calculations|publisher=Babes-Bolyai University lecture notes}})</ref>
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| : <math>\mu_B = \frac{e \hbar}{2 m_e} = 1/2</math> a.u.
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| and in Gaussian-based atomic units,<ref>{{cite book|last1=Budker|first1=Dmitry|last2=Kimball|first2=Derek F.|last3=DeMille|first3=David P.|title=Atomic Physics: An Exploration through Problems and Solutions|year=2004|publisher=Oxford University Press|isbn=978-0-19-850950-9|page=380|url=http://books.google.com/books?id=GW6pclAk-JcC&pg=PA380}}</ref>
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| : <math>\mu_B = \frac{e \hbar}{2 m_e c}=\alpha/2\approx 3.6\times 10^{-3}</math> a.u.
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| == Bohr model in atomic units ==
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| Atomic units are chosen to reflect the properties of electrons in atoms. This is particularly clear from the classical [[Bohr model]] of the [[hydrogen atom]] in its [[ground state]]. The ground state electron orbiting the hydrogen nucleus has (in the classical Bohr model):
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| * Orbital velocity = 1
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| * Orbital radius = 1
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| * [[Angular momentum]] = 1
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| * Orbital period = 2π
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| * [[Ionization energy]] = {{frac|1|2}}
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| * Electric field (due to nucleus) = 1
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| * Electrical attractive force (due to nucleus) = 1
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| == Non-relativistic quantum mechanics in atomic units ==
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| The [[Schrödinger equation]] for an electron in SI units is
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| : <math>- \frac{\hbar^2}{2m_e} \nabla^2 \psi(\mathbf{r}, t) + V(\mathbf{r}) \psi(\mathbf{r}, t) = i \hbar \frac{\partial \psi}{\partial t} (\mathbf{r}, t)</math>.
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| The same equation in '''au''' is
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| : <math>- \frac{1}{2} \nabla^2 \psi(\mathbf{r}, t) + V(\mathbf{r}) \psi(\mathbf{r}, t) = i \frac{\partial \psi}{\partial t} (\mathbf{r}, t)</math>.
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| For the special case of the electron around a hydrogen atom, the [[Hamiltonian (quantum mechanics)|Hamiltonian]] in SI units is:
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| : <math>\hat H = - {{{\hbar^2} \over {2 m_e}}\nabla^2} - {1 \over {4 \pi \epsilon_0}}{{e^2} \over {r}}</math>,
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| while '''atomic units''' transform the preceding equation into
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| : <math>\hat H = - {{{1} \over {2}}\nabla^2} - {{1} \over {r}}</math>.
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| == Comparison with Planck units ==
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| Both [[Planck units]] and au are derived from certain fundamental properties of the physical world, and are free of [[anthropocentric]] considerations. It should be kept in mind that au were designed for atomic-scale calculations in the present-day universe, while Planck units are more suitable for [[quantum gravity]] and early-universe [[physical cosmology|cosmology]]. Both au and Planck units normalize the [[reduced Planck constant]]. Beyond this, Planck units normalize to 1 the two fundamental constants of [[general relativity]] and cosmology: the [[gravitational constant]] ''G'' and the [[speed of light]] in a vacuum, ''c''. Atomic units, by contrast, normalize to 1 the mass and charge of the electron, and, as a result, the speed of light in atomic units is a large value, <math>1/\alpha \approx 137</math>. The orbital velocity of an electron around a small atom is of the order of 1 in atomic units, so the discrepancy between the velocity units in the two systems reflects the fact that electrons orbit small atoms much slower than the speed of light (around 2 orders of magnitude slower).
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| There are much larger discrepancies in some other units. For example, the unit of mass in atomic units is the mass of an electron, while the unit of mass in Planck units is the [[Planck mass]], a mass so large that if a single particle had that much mass it might collapse into a [[black hole]]. Indeed, the Planck unit of mass is 22 orders of magnitude larger than the au unit of mass. Similarly, there are many orders of magnitude separating the Planck units of energy and length from the corresponding atomic units.
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| == See also ==
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| * [[Natural units]]
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| * [[Planck units]]
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| * [[Centimetre gram second system of units#Various extensions of the CGS system to electromagnetism|Various extensions of the CGS system to electromagnetism]].
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| == Notes and references ==
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| * {{cite journal
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| |last1=Shull | first1=H.
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| |last2=Hall | first2=G. G.
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| |year=1959
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| |title=Atomic Units
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| |journal=[[Nature (journal)|Nature]]
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| |volume=184 |issue=4698 |page=1559
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| |doi=10.1038/1841559a0
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| |bibcode = 1959Natur.184.1559S }}
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| {{Reflist|2}}
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| == External links ==
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| * [http://physics.nist.gov/cuu/Constants/index.html CODATA Internationally recommended values of the Fundamental Physical Constants.]
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| {{SI units}}
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| {{Systems of measurement}}
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| [[Category:Systems of units]]
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| [[Category:Natural units]]
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| [[Category:Atomic physics]]
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