Atomic units

Template:Short description Template:Distinguish The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested and named by the physicist Douglas Hartree.<ref name="Hartree1928">Template:Citation</ref> Atomic units are often abbreviated "a.u." or "au", not to be confused with similar abbreviations used for astronomical units, arbitrary units, and absorbance units in other contexts.

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">Template:Cite book</ref>Template:Rp

<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 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 Template:Tmath, Template:Tmath, Template:Tmath, and Template:Tmath all correspond to the value Template:Tmath (see Template:Slink below). The distances relevant to the physics expressed in SI units are naturally on the order of Template:Tmath, while expressed in atomic units distances are on the order of Template:Tmath (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.

History

Hartree defined units based on three physical constants:<ref name="Hartree1928"/>Template:Rp

Both in order to eliminate various universal constants from the equations and also to avoid high powers of 10 in numerical work, it is convenient to express quantities in terms of units, which may be called 'atomic units', defined as follows:

Unit of length, Template:Tmath, on the orbital mechanics the radius of the 1-quantum circular orbit of the H-atom with fixed nucleus.

Unit of charge, Template:Tmath, the magnitude of the charge on the electron.

Unit of mass, Template:Tmath, the mass of the electron.

Consistent with these are:

Unit of action, Template:Tmath.

Unit of energy, Template:Tmath [...]

Unit of time, Template:Tmath.Template:Br {{#if:D.R. HartreeThe Wave Mechanics of an Atom with a Non-Coulomb Central Field. Part I. Theory and Methods|{{#if:|}}

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Here, the modern equivalent of Template:Tmath is the Rydberg constant Template:Tmath, of Template:Tmath is the electron mass Template:Tmath, of Template:Tmath is the Bohr radius Template:Tmath, and of Template:Tmath is the reduced Planck constant Template:Tmath. Hartree's expressions that contain Template:Tmath differ from the modern form due to a change in the definition of Template:Tmath, as explained below.

In 1957, Bethe and Salpeter's book Quantum mechanics of one-and two-electron atoms<ref>Template:Cite book</ref> built on Hartree's units, which they called atomic units abbreviated "a.u.". They chose to use Template:Tmath, their unit of action and angular momentum in place of Hartree's length as the base units. They noted that the unit of length in this system is the radius of the first Bohr orbit and their velocity is the electron velocity in Bohr's model of the first orbit.

In 1959, Shull and Hall<ref name="ShullHall1959"> Template:Cite journal</ref> advocated atomic units based on Hartree's model but again chose to use Template:Tmath as the defining unit. They explicitly named the distance unit a "Bohr radius"; in addition, they wrote the unit of energy as Template:Tmath and called it a Hartree. These terms came to be used widely in quantum chemistry.<ref>Template:Cite book</ref>Template:Rp

In 1973 McWeeny extended the system of Shull and Hall by adding permittivity in the form of Template:Tmath as a defining or base unit.<ref name="McWeeny1973">Template:Cite journal</ref><ref name="JerrardMcNeill1992">Template:Cite book</ref> Simultaneously he adopted the SI definition of Template:Tmath so that his expression for energy in atomic units is Template:Tmath, matching the expression in the 8th SI brochure.<ref>Template:SIbrochure8th. Note that this information is omitted in the 9th edition.</ref>

Definition

A set of base units in the atomic system as in one proposal are the electron rest mass, the magnitude of the electronic charge, the Planck constant, and the permittivity.<ref name="McWeeny1973"/>Template:Refn In the atomic units system, each of these takes the value 1; the corresponding values in the International System of Units<ref name="Brochure9_2019"> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Rp are given in the table.

Base atomic unitsTemplate:Cref

Symbol and Name	Quantity (dimensions)Template:Cref	Atomic unitsTemplate:Cref	SI units
Template:Tmath, reduced Planck constant	action (ML2T−1)	1	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
Template:Tmath, elementary charge	charge (Q)	1	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
Template:Tmath, electron rest mass	mass (M)	1	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
Template:Tmath, permittivity	permittivity (Q2W−1L−1)	1	Template:Physconst <ref name="auperm"> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>

Table notes

• Template:Cnote
• Template:Cnote
• Template:Cnote

Units

Three of the defining constants (reduced Planck constant, elementary charge, and electron rest mass) are atomic units themselves – of action,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> electric charge,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and mass,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> respectively. Two named units are those of length (Bohr radius Template:Tmath) and energy (hartree Template:Tmath).

Defined atomic units

Atomic unit of	Expression	Value in SI units	Other equivalents
electric charge density	<math> e/a_0^3 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric current	<math> e E_\text{h} / \hbar </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric charge	<math>e</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric dipole moment	<math> e a_0 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Math Template:Val
electric quadrupole moment	<math> e a_0^2 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric potential	<math> E_\text{h} / e </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric field	<math>E_\text{h} / e a_0 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
electric field gradient	<math>E_\text{h} / e a_0^2 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
permittivity	<math> e^2 / a_0 E_\text{h} </math>	Template:Physconst <ref name="auperm"> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Tmath
electric polarizability	<math> e^2 a_0^2 / E_\text{h} </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
1st hyperpolarizability	<math>e^3 a_0^3 / E_\text{h}^2</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
2nd hyperpolarizability	<math>e^4 a_0^4 / E_\text{h}^3</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
magnetic dipole moment	<math> \hbar e / m_\text{e} </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Tmath
magnetic flux density	<math> \hbar/e a_0^2 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Math Template:Val
magnetizability	<math> e^2 a_0^2 / m_\text{e}</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
action	<math>\hbar</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
energy	<math> E_\text{h} </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Tmath, Template:Tmath, Template:Val <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
force	<math>E_\text{h} / a_0 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Val, Template:Val
length	<math> a_0 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Tmath, Template:Val
mass	<math>m_\text{e}</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
momentum	<math> \hbar/a_0 </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
time	<math>\hbar / E_\text{h}</math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
velocity	<math> a_0 E_\text{h} / \hbar </math>	Template:Physconst <ref> {{#invoke:citation/CS1|citation	CitationClass=web }}</ref>	Template:Tmath
Template:Tmath: speed of light, Template:Tmath: vacuum permittivity, Template:Tmath: Rydberg constant, Template:Tmath: Planck constant, Template:Tmath: fine-structure constant, Template:Tmath: Bohr magneton, Template:Math: correspondence

Conventions

Different conventions are adopted in the use of atomic units, which vary in presentation, formality and convenience.

Explicit units

• Many texts (e.g. Jerrard & McNiell,<ref name="JerrardMcNeill1992"/> Shull & Hall<ref name="ShullHall1959"/>) define the atomic units as quantities, without a transformation of the equations in use. As such, they do not suggest treating either quantities as dimensionless or changing the form of any equations. This is consistent with expressing quantities in terms of dimensional quantities, where the atomic unit is included explicitly as a symbol (e.g. Template:Tmath, Template:Tmath, or more ambiguously, Template:Tmath), and keeping equations unaltered with explicit constants.<ref name="Pilar2001">Template:Cite book</ref>
• Provision for choosing more convenient closely related quantities that are more suited to the problem as units than universal fixed units are is also suggested, for example based on the reduced mass of an electron, albeit with careful definition thereof where used (for example, a unit Template:Tmath, where Template:Tmath for a specified mass Template:Tmath).<ref name="ShullHall1959"/>

A convention that eliminates units

In atomic physics, it is common to simplify mathematical expressions by a transformation of all quantities:

• Hartree suggested that expression in terms of atomic units allows us "to eliminate various universal constants from the equations", which amounts to informally suggesting a transformation of quantities and equations such that all quantities are replaced by corresponding dimensionless quantities.<ref name="Hartree1928"/>Template:Rp He does not elaborate beyond examples.
• McWeeny suggests that "... their adoption permits all the fundamental equations to be written in a dimensionless form in which constants such as Template:Tmath, Template:Tmath and Template:Tmath are absent and need not be considered at all during mathematical derivations or the processes of numerical solution; the units in which any calculated quantity must appear are implicit in its physical dimensions and may be supplied at the end." He also states that "An alternative convention is to interpret the symbols as the numerical measures of the quantities they represent, referred to some specified system of units: in this case the equations contain only pure numbers or dimensionless variables; ... the appropriate units are supplied at the end of a calculation, by reference to the physical dimensions of the quantity calculated. [This] convention has much to recommend it and is tacitly accepted in atomic and molecular physics whenever atomic units are introduced, for example for convenience in computation."
• An informal approach is often taken, in which "equations are expressed in terms of atomic units simply by setting Template:Tmath".<ref name="Pilar2001"/><ref>Template:Cite book</ref><ref>Template:Cite book</ref> This is a form of shorthand for the more formal process of transformation between quantities that is suggested by others, such as McWeeny.

Physical constants

Dimensionless physical constants retain their values in any system of units. Of note is the fine-structure constant Template:Tmath, 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, is Template:Tmath<ref name="KarplusPorter1970">Template:Citation</ref>Template:Rp

Some physical constants expressed in atomic units

Name	Symbol/Definition	Value in atomic units
speed of light	<math>c</math>	<math>(1/\alpha) \,a_0 E_\text{h}/\hbar \approx 137 \,a_0 E_\text{h}/\hbar</math>
classical electron radius	<math>r_\text{e}=\frac{1}{4\pi\epsilon_0}\frac{e^2}{m_\text{e} c^2}</math>	<math>\alpha^2 \,a_0 \approx 0.0000532 \,a_0</math>
reduced Compton wavelength of the electron	Template:Math <math>= \frac{\hbar}{m_\text{e} c}</math>	<math>\alpha \,a_0 \approx 0.007297 \,a_0</math>
proton mass	<math>m_\text{p}</math>	<math>\approx 1836 \,m_\text{e}</math>

Bohr model in atomic units

Atomic units are chosen to reflect the properties of electrons in atoms, which is particularly clear in the classical Bohr model of the hydrogen atom for the bound electron in its ground state:

• Mass = 1 a.u. of mass
• Charge = −1 a.u. of charge
• Orbital radius = 1 a.u. of length
• Orbital velocity = 1 a.u. of velocity<ref name="KarplusPorter1970"/>Template:Rp
• Orbital period = 2π a.u. of time
• Orbital angular velocity = 1 radian per a.u. of time
• Orbital momentum = 1 a.u. of momentum
• Ionization energy = Template:Sfrac a.u. of energy
• Electric field (due to nucleus) = 1 a.u. of electric field
• Lorentz force (due to nucleus) = 1 a.u. of force

References

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