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Atomic units (au or a.u.) form a system of natural units which is especially convenient for atomic physics calculations. There are two different kinds of atomic units, 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, where the numerical values of the following four fundamental physical constants are all unity by definition: * Electron mass ; * Elementary charge ; * Reduced Planck's constant ; * Coulomb's constant . In Hartree units, the speed of light is approximately . 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 different contexts. == 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: * "". This is the clearest notation (but least common), where the atomic unit is included explicitly as a symbol. * "" ("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, "" The dimension needs to be inferred from context.〔 * "". 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 , so . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「atomic units」の詳細全文を読む スポンサード リンク
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