Electric dipole approximation breakdown

Breakdown of the electric dipole approximation. The expression for the transition probability for spontaneous emission of electric dipole radiation in a one-electron atom. 

Breakdown of the Bom-Oppenheimer approximation is responsible for the small but non-zero permanent electric dipole moment of HD (2 x 10 Cm, Trefler and Gush, 1968) but otherwise the effect is negligible to chemical accuracy. 

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