Editing Atomic units (section)

=== 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"/>{{rp|91}}  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 {{tmath|1= e }}, {{tmath|1= m }} and {{tmath|1= h }} 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 {{tmath|1= \hbar = m_\text{e} = e = 4 \pi \epsilon_0 = 1}}".<ref name="Pilar2001"/><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=https://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=https://books.google.com/books?id=Jj-ad_2aNOAC&pg=PA5 }}</ref>  This is a form of shorthand for the more formal process of transformation between quantities that is suggested by others, such as McWeeny.