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I read the follow paper about van del waals interaction in quantum mechanics

http://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/van_der_waals.pdf

In this paper the potential

[tex]V= e^2/R + e^2/(R+y)+e^2/(R-x)+ e^2/(R+y-x) [/tex]

is aproximated to [tex]V \approx -2 e^2/R^3 xy[/tex]

with R>>|x|,|y|

why this paper use this aproximation?

if i use Taylor series, my aproximation is

[tex]V=4 e^2 /R + 2 e^2/R (x-y) + 2 e^2 /R^3 (x^2+y^2) -2 e^2/R^3 xy[/tex]