1. We call quantum physics an ab initio model because it deals with a set of natural phenomenon. Also, the basic principles of quantum physics are irreplaceable. We can also use the equations of quantum physics to predict new phenomena.
2. Same as explained above, classical mechanics deals with the laws of motion and to describe the trajectory of objects, so it could also be called an ab initio model.
3. To describe the electromagnetic phenomenon, we can use the Maxwell’s equations. So, we can call classical electromagnetism an ab initio model.