Coupling constants 1.108 radiative corrections

The leading quantum electrodynamic effects to be accounted for in electronic structure calculations are the radiative corrections known as electron self-energy interaction and vacuum polarization. For the energy of electronic systems, the latter is usually small compared to the former, but only the latter can be expressed in terms of an effective additive potential to be included in the electronic structure calculations. The total vacuum polarization potential can be expanded into a double power series in the fine structure constant a and the external coupling constant Za. The lowest-order term, the Uehling potential, can be expressed as [110-112] ... 

Differently stated, the fine structure constant is altered by radiative corrections and a(m ) is the effective electromagnetic coupling at the ft mass. This concept is explained in detail in Chapters 20 and 21. 

The simplest radiative correction is due to ff self-energy loops in the photon propagator. Their effect can be taken into account by defining a running or effective coupling constant a ) which depends upon the carried by the photon line when it couples to a charged fermion. (This concept is treated in Section 21.7.) Thus one should use... 

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