Organic molecular system is an important subject of study in a variety of research fields including materials science and biology. As one of the most important computer simulation tools, molecular mechanics (MM) plays an essential role in the studies of organic materials and biomolecules. All MM simulations rely on a potential energy surface (PES), which describes the energies and the forces of the simulated atoms. For the sake of computational efficiency, the PES is typically approximated using a classical model, namely molecular force field, instead of being computed ab initioly on-the-fly. Therefore, the quality of the underlying force field limits the accuracy and the predictive power of the MM simulation.