Potential Energy of two charges which are not fixed

Suppose I have 2 charges (+q and +Q) which remain fixed and are placed at a seperation R. They will have some Potential Energy which is kQq/R.

So I wanted to ask that if the charges are not fixed then they will exert repulsive forces on each other and due this Force they will move away from each other right?

So how could we define potential energy of the charges if they are not fixed because due to the Electrostatic Force the distance (displacement) between them will change until the force is 0 which will be at infinity?

Please help.