The artificial potential method always takes robot as a point, if the manipulator is 6 degrees of freedom, how to use this method in path planning while six links can avoid the collision.
The artificial potential function correctly treats the robot as a point. It is the end-effector that is to be considered as the point object to be planned for while the rest of the manipulator kinematic chain is only considered to determine the free configuration space.
When the planning algorithm determines an end-effector's cartesian coordinate, you should employ collision checking algorithms (like the Gilbert–Johnson–Keerthi distance algorithm available in open-source packages like Flexible Collision Library(FCL) or Bullet) to determine if the pose lies in the free configuration space or not. If the pose belongs to C-free, then it is added to the path else a new point in the vicinity is proposed and same steps are repeated.