# Perturbation approach to solve attractive part of Lennard-Jones potential

#### Torgny

The text is taken from this site: http://www.chem.konan-u.ac.jp/PCSI/web_material/LJ.pdf

I don't get the definition added just after (7). Where is it taken from and what does it mean? Why is it the energy? And can someone show the arithmetics and the fundamental principles on how to get from one to the other line in the two lines just before (8). Both questions are underlined in the text above.