Answer
The nuclei of the two atoms are being forced too close together, which causes an increase in energy between the two atoms.
Work Step by Step
At the minimum, the attractive and repulsive forces are balanced. However, as the Cl-Cl bond distance decreases, the nuclei are being forced closer together, leading to an increase in repulsive force between the positively-charged nuclei. This increase in repulsive force leads to an increase in energy.
