Van del waals interaction in qm

[alejandrito29]

Hello

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

$$V= e^2/R + e^2/(R+y)+e^2/(R-x)+ e^2/(R+y-x)$$

is aproximated to $$V \approx -2 e^2/R^3 xy$$

with R>>|x|,|y|

why this paper use this aproximation?

if i use Taylor series, my aproximation is

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

 

[alejandrito29]

ohh, sorry, my problem is with signus of V...delete my post please