Firstly if we understand, what is an ab initio model then it comes to point that ab initio is representing the first principle mean the fundamental equations.

Now the Quantum physics, is now a days a fundamental importance almost in every area of physics so that we called quantum physics an ab initio model.

Similarly in the classical physics, the newton and Lagrange equations, mainly presenting the whole classical physics and through which every macroscopic phenomena going through experience that’s why due to the newton and Lagrange consequently the classical physics called the ab initio model.

Yes, the classical electrodynamics also representing the ab initio model due to the presences of four fundamental Maxwell’s equations in classical frame.