Quantum Physics can be called an ab initio model as it takes Schrodinger equation as the natural law and can predict a lot of natural phenomena in the microscopic domain.
Classical mechanics is also an ab initio model as it takes a large macroscopic system and predict their behaviour(such as trajectory of the planets, motion of a projectile, motion of an uniformly rotating disk etc.) just using Newton’s equation of motion.
Electromagnetism is the simplest beautiful laws of nature which can explain electromagnetic phenomena very well and accurately using Maxwell’s equations. So it can also be called an ab initio model.