Atkins & de Paula:
Physical Chemistry for the Life Sciences
The Lennard-Jones potential is an approximation to the true intermolecular potential energy curves. It models the attractive component by a contribution that is proportional to 1/r6, and the repulsive component by a contribution that is proportional to 1/r12. Specifically, these choices result in the Lennard-Jones (12,6)-potential. Although there are good theoretical reasons for the former, there is plenty of evidence to show that 1/r12 is only a very poor approximation to the repulsive part of the curve.
The formula is
Where 
(kJ mol-1) is the depth of the well, 
(pm)
the separation at which V = 0, and r is the range of separations in picometers.
To graph the potential (kJ mol-1) enter the parameters and hit New Plot.
Up to 5 plots can be displayed at one time. The Clear button will remove all plots. To see the parameters
for each plot hit the Legend on/off button. The Redraw button will refresh the graph. This is useful when
the function domain (i.e., r ) has been changed. To see the value of each plot at a given point, move
your cursor to the desired location then click and hold.