We present a derivation of an ab intio model potential (AIMP) based on the Van Vleck Perturbation theory. We applied the derivation to the specific case of a molecular system made of one alkali atom interacting with rare gas atoms. Our approach provides a formal background for the empirical potential often used to study this kind of molecular systems and allows us to discuss their intrinsic limitations and some possible improvements. In particular, the use of AIMP, which keeps the nodal structure of the orbitals, allows us to take into account accurately the spin-orbit relativistic correction. Its application to alkali-rare gas diatomic molecules allows us to reproduce rather well the known experimental and the best ab initio calculations at a lower computational cost.