ab intito

quantum physics and the schrodinger equation is based on the principle of least action and the particle-wave duality . Hence it is an ab initio method because it originates from a very broad principle that is applicable in a large set of phenomena ( except relativistic situations).
As for classical mechanics, if our system is macroscopic then yes it is an ab initio method but since this is an approximation we can’t expect it to explain all possible macroscopic phenomena. For example classical mechanics can’t explain magnetism.
i don’t think classical electromagnetism is an ab initio model .