Theoretically, according to the Born-Oppenheimer approximation, the potential energy values of any configuration spanned in chemically relevant regions for a target molecular system can be determined by various electronic structure methods. With potential energies of ample configurations for a specified molecular system, fitting or interpolating techniques is employed to construct a multidimensional function of nuclear configurations. This multi-dimensional function is called the PES of the target system. The PES discussed in this work is restricted to be full-dimensional and accurate (all were based on the CCSD(T) or MRCI calculations), although reduced-dimensional PESs may be efficient and accurate in some scattering or spectroscopy computations. Note that BO approximation may fail at where energies of different electronic states are close even degenerate. For such cases, the concepts of the BO approximation and the PESs remain useful. Nonadiabatically coupled PESs can be derived in a diabatic representation with the coupling given as the off-diagonal elements of a potential energy matrix. Fitting PESs of different electronic states and their couplings in a diabatic potential energy matrix is not discussed further in this work. Relevant excellent reviews and work are available. Then various properties can be determined by running either classical or quantum dynamic calculations on the PES of the system. Consequently, although PES can't be measured directly by experiments, its performance significantly affects the dynamic outcome. As stressed in our previous review article, "an inaccurate PES is not expected to yield correct results no matter how accurate the dynamical method might be. … the lack of accurate global PESs represented the main stumbling block in our understanding of reaction dynamics." The energies and its derivatives can be also determined on the fly. This so-called direct dynamics approach is popular for exploring the potential landscape of a complex system without the need to construct a full-dimensional PES.