Atomic units (au) form a system of units convenient for atomic physics, electromagnetism, and quantum electrodynamics, especially when the focus is on the properties of electrons. There are two different kinds of atomic units, which one might name Hartree atomic units and Rydberg atomic units, which differ in the choice of the unit of mass and charge. This article deals with Hartree atomic units. In au, the numerical values of the following six physical constants are all unity by definition:
- Two properties of the electron, its mass and charge;
- Two properties of the hydrogen atom, its Bohr radius and the absolute value of its electric potential energy in the ground state;
- Two constants, reduced Planck's constant and that for Coulomb's Law.
Not to be confused with Astronomical Units (also abbreviated to 'au').
Contents |
Fundamental units
| Fundamental Atomic Units | ||||
|---|---|---|---|---|
| Quantity | Name | Symbol | SI value | Planck unit scale |
| mass | electron rest mass |  |
9.109 3826(16)×10-31 kg | 10-8 kg |
| length | Bohr radius |  |
5.291 772 108(18)×10-11 m | 10-35 m |
| charge | elementary charge |  |
1.602 176 53(14)×10-19 C | 10-18 C |
| angular momentum | Reduced Planck's constant |  |
1.054 571 68(18)×10-34 J s | (same) |
| energy | Hartree energy |  |
4.359 744 17(75)×10-18 J | 109 J |
These five quantities are not independent; to normalize all five quantities to 1, it suffices to normalize any four of them to 1. The normalization of the Hartree energy is only an incidental consequence of normalizing the other four quantities.
Some derived units
| Derived Atomic Units | |||
|---|---|---|---|
| Quantity | Expression | SI value | Planck unit scale |
| time |  |
2.418 884 326 505(16)×10-17 s | 10-43 s |
| velocity |  |
2.187 691 2633(73)×106 m s-1 | 108 m s-1 |
| force |  |
8.238 7225(14)×10-8 N | 1044 N |
| temperature |  |
3.157 7464(55)×105 K | 1032 K |
| pressure |  |
2.942 1912(19)×1013 N m-2 | 10114 Pa |
Comparison with Planck units
Both Planck units and au are derived from certain fundamental properties of the physical world, and are free of anthropocentric considerations. To facilitate comparing the two systems of units, the above tables show the order of magnitude, in SI units, of the Planck unit corresponding to each atomic unit. Generally, when an atomic unit is "large" in SI terms, the corresponding Planck unit is "small", and vice versa. 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 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. Letting α denote the fine structure constant, the au value of c is α-1 ≈ 137.036.
Atomic units, by contrast, normalize to 1 the mass and charge of the electron, and a0, the Bohr radius of the hydrogen atom. Normalizing a0 to 1 amounts to normalizing the Rydberg constant, R∞, to 4π/α = 4πc. Given au, the Bohr magneton is μB=1/2. The corresponding Planck value is e/2me. Finally, au normalize the Hartree energy to 1, while Planck units normalize to 1 Boltzmann's constant k, which relates energy and temperature.
Quantum mechanics and electrodynamics simplified
The (non-relativistic) Schrödinger equation for an electron in SI units is
.
The same equation in au is
.
For the special case of the electron around a hydrogen atom, the Hamiltonian in SI units is:
,
while atomic units transform the preceding equation into
.
Finally, Maxwell's equations take the following elegant form in au:
Extending the atomic units to electromagnitsm, there is some ambiguity in defining the atomic unit of magnetic field, electric current, etc. The above Maxwell equations use the "Gaussian" convention, in which a plane wave has electric and magnetic fields of equal magnitude. This is equivalent to the extension of the CGS units to electromagnetism.
See also
References
- H. Shull and G. G. Hall, Atomic Units, Nature, volume 184, no. 4698, page 1559 (Nov. 14, 1959)
- G. Drake (ed.), Springer Handbook of Atomic, Molecular, and Optical Physics. Springer, 2nd ed., 2006
External links
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