Atomic Units
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Atomic Units

The Hartree atomic units are a system of natural units of measurement which is especially convenient for atomic physics and computational chemistry calculations. They are named after the physicist Douglas Hartree.[1] In this system the numerical values of the following four fundamental physical constants are all unity by definition:

• Reduced Planck constant: ${\displaystyle \hbar =1}$, also known as the atomic unit of action[2]
• Elementary charge: ${\displaystyle e=1}$, also known as the atomic unit of charge[3]
• Bohr radius: ${\displaystyle a_{0}=1}$, also known as the atomic unit of length[4]
• Electron mass: ${\displaystyle m_{\text{e}}=1}$, also known as the atomic unit of mass[5][6]

In Hartree atomic units, the speed of light is approximately atomic units of velocity. Atomic units are often abbreviated "a.u." or "au", not to be confused with the same abbreviation used also for astronomical units, arbitrary units, and absorbance units in other contexts.

## Defining constants

Each unit in this system can be expressed as a product of powers of four physical constants without a multiplying constant. This makes it a coherent system of units, as well as making the numerical values of the defining constants in atomic units equal to unity.

Defining constants
Name Symbol Value in SI units
reduced Planck constant ${\displaystyle \hbar }$ [7]
elementary charge ${\displaystyle e}$ [8]
Bohr radius ${\displaystyle a_{0}}$ [9]
electron rest mass ${\displaystyle m_{\mathrm {e} }}$ [10]

Five symbols are commonly used as units in this system, only four of them being independent:[11]:94-95

Constants used as units
Dimension Symbol Definition
action ${\displaystyle \hbar }$ ${\displaystyle \hbar }$
electric charge ${\displaystyle e}$ ${\displaystyle e}$
length ${\displaystyle a_{0}}$ ${\displaystyle 4\pi \epsilon _{0}\hbar ^{2}/(m_{\text{e}}e^{2})}$
mass ${\displaystyle m_{\text{e}}}$ ${\displaystyle m_{\text{e}}}$
energy ${\displaystyle E_{\text{h}}}$ ${\displaystyle \hbar ^{2}/(m_{\text{e}}a_{0}^{2})}$

## Units

Below are listed units that can be derived in the system. A few are given names, as indicated in the table.

Derived atomic units
Atomic unit of Name Expression Value in SI units Other equivalents
1st hyperpolarizability ${\displaystyle e^{3}a_{0}^{3}/E_{\text{h}}^{2}}$ [12]
2nd hyperpolarizability ${\displaystyle e^{4}a_{0}^{4}/E_{\text{h}}^{3}}$ [13]
action ${\displaystyle \hbar }$ [14]
charge ${\displaystyle e}$ [15]
charge density ${\displaystyle e/a_{0}^{3}}$ [16]
current ${\displaystyle eE_{\text{h}}/\hbar }$ [17]
electric dipole moment ${\displaystyle ea_{0}}$ [18] ?
electric field ${\displaystyle E_{\text{h}}/(ea_{0})}$ [19] ,
electric field gradient ${\displaystyle E_{\text{h}}/(ea_{0}^{2})}$ [20]
electric polarizability ${\displaystyle e^{2}a_{0}^{2}/E_{\text{h}}}$ [21]
electric potential ${\displaystyle E_{\text{h}}/e}$ [22]
electric quadrupole moment ${\displaystyle ea_{0}^{2}}$ [23]
energy hartree ${\displaystyle E_{\text{h}}}$ [24] ${\displaystyle 2R_{\infty }hc}$, ${\displaystyle \alpha ^{2}m_{\text{e}}c^{2}}$,
force ${\displaystyle E_{\text{h}}/a_{0}}$ [25] ,
length bohr ${\displaystyle a_{0}}$ [26] ${\displaystyle \hbar /(m_{\text{e}}c\alpha )}$,
magnetic dipole moment ${\displaystyle e\hbar /m_{\text{e}}}$ [27] ${\displaystyle 2\mu _{\text{B}}}$
magnetic flux density ${\displaystyle \hbar /(ea_{0}^{2})}$ [28] ?
magnetizability ${\displaystyle e^{2}a_{0}^{2}/m_{\text{e}}}$ [29]
mass ${\displaystyle m_{\mathrm {e} }}$ [30]
momentum ${\displaystyle \hbar /a_{0}}$ [31]
permittivity ${\displaystyle e^{2}/(a_{0}E_{\text{h}})}$ [32] ${\displaystyle 4\pi \epsilon _{0}}$
pressure ${\displaystyle E_{\text{h}}/{a_{0}}^{3}}$
time ${\displaystyle \hbar /E_{\text{h}}}$ [33]
velocity ${\displaystyle a_{0}E_{\text{h}}/\hbar }$ [34] ${\displaystyle \alpha c}$

Here,

${\displaystyle c}$ is the speed of light
${\displaystyle \epsilon _{0}}$ is the vacuum permittivity
${\displaystyle R_{\infty }}$ is the Rydberg constant
${\displaystyle h}$ is the Planck constant
${\displaystyle \alpha }$ is the fine-structure constant
${\displaystyle \mu _{\text{B}}}$ is the Bohr magneton
? denotes correspondence between quantities since equality does not apply.

## Use and notation

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.

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:

• "${\displaystyle m=3.4~m_{\text{e}}}$". This is the clearest notation (but least common), where the atomic unit is included explicitly as a symbol.[35]
• "${\displaystyle m=3.4~{\text{a.u.}}}$" ("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, "${\displaystyle L=3.4~{\text{a.u.}}}$" The dimension must be inferred from context.[35]
• "${\displaystyle m=3.4}$". 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 ${\displaystyle m_{\text{e}}=1}$, so ${\displaystyle 3.4~m_{\text{e}}=3.4}$.[36][37]

## Physical constants

Dimensionless physical constants retain their values in any system of units. Of note is the fine-structure constant ${\displaystyle \alpha ={\frac {e^{2}}{(4\pi \epsilon _{0})\hbar c}}\approx 1/137}$, which appears in expressions as a consequence of the choice of units. For example, the numeric value of the speed of light, expressed in atomic units, has a value related to the fine structure constant.

Some physical constants expressed in atomic units
Name Symbol/Definition Value in atomic units
speed of light ${\displaystyle c}$ ${\displaystyle (1/\alpha )\,a_{0}E_{\text{h}}/\hbar \approx 137\,a_{0}E_{\text{h}}/\hbar }$
classical electron radius ${\displaystyle r_{\mathrm {e} }={\frac {1}{4\pi \epsilon _{0}}}{\frac {e^{2}}{m_{\mathrm {e} }c^{2}}}}$ ${\displaystyle \alpha ^{2}\,a_{0}\approx 0.0000532\,a_{0}}$
reduced Compton wavelength
of the electron
?e ${\displaystyle ={\frac {\hbar }{m_{\text{e}}c}}}$ ${\displaystyle \alpha \,a_{0}\approx 0.007297\,a_{0}}$
Bohr radius ${\displaystyle a_{0}={\frac {4\pi \epsilon _{0}\hbar ^{2}}{m_{\text{e}}e^{2}}}}$ ${\displaystyle 1\,a_{0}}$
proton mass ${\displaystyle m_{\mathrm {p} }}$ ${\displaystyle m_{\mathrm {p} }\approx 1836\,m_{\text{e}}}$

## Bohr model in atomic units

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):

• Mass = 1 a.u. of mass
• Orbital radius = 1 a.u. of length
• Orbital velocity = 1 a.u. of velocity
• Orbital period = 2? a.u. of time
• Orbital angular velocity = 1 radian per a.u. of time
• Orbital angular momentum = 1 a.u. of momentum
• Ionization energy = a.u. of energy
• Electric field (due to nucleus) = 1 a.u. of electric field
• Electrical attractive force (due to nucleus) = 1 a.u. of force

## Non-relativistic quantum mechanics in atomic units

The Schrödinger equation for an electron in SI units is

${\displaystyle -{\frac {\hbar ^{2}}{2m_{\text{e}}}}\nabla ^{2}\psi (\mathbf {r} ,t)+V(\mathbf {r} )\psi (\mathbf {r} ,t)=i\hbar {\frac {\partial \psi }{\partial t}}(\mathbf {r} ,t)}$.

The same equation in atomic units is

${\displaystyle -{\frac {1}{2}}\nabla ^{2}\psi (\mathbf {r} ,t)+V(\mathbf {r} )\psi (\mathbf {r} ,t)=i{\frac {\partial \psi }{\partial t}}(\mathbf {r} ,t)}$.

For the special case of the electron around a hydrogen atom, the Hamiltonian in SI units is:

${\displaystyle {\hat {H}}=-{{{\hbar ^{2}} \over {2m_{\text{e}}}}\nabla ^{2}}-{1 \over {4\pi \epsilon _{0}}}{{e^{2}} \over {r}}}$,

while atomic units transform the preceding equation into

${\displaystyle {\hat {H}}=-{{{1} \over {2}}\nabla ^{2}}-{{1} \over {r}}}$.

## Comparison with Planck units

Both Planck units and atomic units are derived from certain fundamental properties of the physical world, and have little anthropocentric arbitrariness, but do still involve some arbitrary choices in terms of the defining constants. Atomic units were designed for atomic-scale calculations in the present-day universe, while Planck units are more suitable for quantum gravity and early-universe cosmology. Both atomic units 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 ${\displaystyle G}$ and the speed of light in vacuum, ${\displaystyle c}$. 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, ${\displaystyle 1/\alpha \approx 137}$. 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 by around 2 orders of magnitude more slowly than the speed of light.

There are much larger differences for 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. The Planck unit of mass is 22 orders of magnitude larger than the atomic unit of mass. Similarly, there are many orders of magnitude separating the Planck units of energy and length from the corresponding atomic units.

## Notes and references

• Shull, H.; Hall, G. G. (1959). "Atomic Units". Nature. 184 (4698): 1559. Bibcode:1959Natur.184.1559S. doi:10.1038/1841559a0.
1. ^ Hartree, D. R. (1928). "The Wave Mechanics of an Atom with a Non-Coulomb Central Field. Part I. Theory and Methods". Mathematical Proceedings of the Cambridge Philosophical Society. 24 (1). Cambridge University Press. pp. 89-110. Bibcode:1928PCPS...24...89H. doi:10.1017/S0305004100011919.
2. ^ "2018 CODATA Value: atomic unit of action". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
3. ^ "2018 CODATA Value: atomic unit of charge". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
4. ^ "2018 CODATA Value: atomic unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
5. ^ "2018 CODATA Value: atomic unit of mass". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
6. ^ Not to be confused with the unified atomic mass unit.
7. ^ "2018 CODATA Value: reduced Planck constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
8. ^ "2018 CODATA Value: elementary charge". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
9. ^ "2018 CODATA Value: Bohr radius". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
10. ^ "2018 CODATA Value: electron mass in u". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
11. ^
12. ^ "2018 CODATA Value: atomic unit of 1st hyperpolarizability". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
13. ^ "2018 CODATA Value: atomic unit of 2nd hyperpolarizability". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
14. ^ "2018 CODATA Value: atomic unit of action". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
15. ^ "2018 CODATA Value: atomic unit of charge". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
16. ^ "2018 CODATA Value: atomic unit of charge density". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
17. ^ "2018 CODATA Value: atomic unit of current". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
18. ^ "2018 CODATA Value: atomic unit of electric dipole moment". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
19. ^ "2018 CODATA Value: atomic unit of electric field". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
20. ^ "2018 CODATA Value: atomic unit of electric field gradient". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
21. ^ "2018 CODATA Value: atomic unit of electric polarizability". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
22. ^ "2018 CODATA Value: atomic unit of electric potential". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
23. ^ "2018 CODATA Value: atomic unit of electric quadrupole moment". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
24. ^ "2018 CODATA Value: atomic unit of energy". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
25. ^ "2018 CODATA Value: atomic unit of force". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
26. ^ "2018 CODATA Value: atomic unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
27. ^ "2018 CODATA Value: atomic unit of magnetic dipole moment". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
28. ^ "2018 CODATA Value: atomic unit of magnetic flux density". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
29. ^ "2018 CODATA Value: atomic unit of magnetizability". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
30. ^ "2018 CODATA Value: atomic unit of mass". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
31. ^ "2018 CODATA Value: atomic unit of momentum". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
32. ^ "2018 CODATA Value: atomic unit of permittivity". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
33. ^ "2018 CODATA Value: atomic unit of time". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
34. ^ "2018 CODATA Value: atomic unit of velocity". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved .
35. ^ a b Pilar, Frank L. (2001). Elementary Quantum Chemistry. Dover Publications. p. 155. ISBN 978-0-486-41464-5.
36. ^ Bishop, David M. (1993). Group Theory and Chemistry. Dover Publications. p. 217. ISBN 978-0-486-67355-4.
37. ^ Drake, Gordon W. F. (2006). Springer Handbook of Atomic, Molecular, and Optical Physics (2nd ed.). Springer. p. 5. ISBN 978-0-387-20802-2.