FORCE / SEPARATION GRAPH
When the forces of attraction and repulsion for two molecules at various separations are added together this is the resultant graph, the is an equalibrium position at r where the particles do not exert any force on each other. At a closer separation a repeling force (taken as +ve) acts on the molecule, at a larger separtion there is a (-ve) attractive force.

POTENTIAL ENERGY / SEPARATION GRAPH
This graph of potentil energy (or work done in bring the molecule to that point from infinity) is found from integrating the above graph of force with respect to distance (from the definition of work). From infinity to r work is done by the particle after r, work has to be done on the particle. Normally a particle in this system has a certain amount of energy, in the form of potential energy due to it position and kinetic energy, if we draw a straightline through the bottum of the graph parallel to the x axis, then the kinetic energy will be equal to the perpendicular distance between this line and the curve. It should be noted that there is a minimum value of potential energy at r, a molecule at this point on the curve is at absolute zero and does not have any K.E. The point where the potential energy is zero (and the curve crosses the x axis) is said to be the distance of closest approach.