[Ifeffit] reference

[Kelly, Shelly D.]

Hello All,

Does anyone know of a reference that explicitly shows that sigma2 for a
linear backscattering path is approximately twice a single scattering
path?

My example is for a uranyl moiety (UO2).  The structure of this moiety
is symmetric and linear with a dumbbell shape with O1=U=O2 all in a row.
I need a reference that does the calculation to show that the 4 leg path
(U-O1-U-O2-U) has a sigma2 value that is twice the single scattering
path (U-O1-U) from one of the oxygen atoms.  

Thanks for your help
Shelly