Angular reduction of the Breit interaction

As shown in detail in Ref. [31], the matrix elements of the first term in (86), the unretarded Breit interaction. 

The mat element of the retardation part of the Brert mteractron takes a similar form, 

Matrix elements of the frequency-dependent Breit interaction 612( 0) are somewhat more complicated they can be evaluated using the formulas given above with the following 

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