Quantum physics are an ab initio model because it starts from the most basic equation that describes fundamental behavior of nature, namely the Shrodinger equations. Solving this equation will give you the properties of the system you study. For classical mechanics the first principles are the laws of Newton. Solving these will also provide the properties such as velocity and acceleration of a certain object. Classical electromagnitism is also an ab initio model for which the first principles are given by the laws of Maxwell.