An interatomic model potential for molecular dynamics is derived from first-principles and used to study the molecular rotations and relaxation times in methylammonium lead halide, here considered the prototypical example of a hybrid crystal with a strong reorientational dynamics. Within the limits of a simple ionic scheme, the potential is able to catch the main qualitative features of the material at zero and finite temperature and opens the way to the development of classical potentials for hybrid perovskites. In agreement with experiments and previous theoretical findings, the molecule trajectories exhibit a transition from a dynamics dominated by high symmetry directions at low temperature to a fast dynamics at room temperature in which the molecule can reorient quasi-randomly. By computing the angular time correlation function we discuss the reorientational time as a function of temperature in comparison with existing literature, providing a simple model and a clear attribution of the relaxation times in terms of their temperature dependence. This work clarifies the temperature dependence of the relaxation times and the interpretation of the experimental data in terms of the different mechanisms contributing to the molecule dynamics.