Static computations solve the electronic Schrödinger equation (eq. 2.6) for a given arrangement of nuclei. Structural optimization then determines the best arrangement of nuclei for a given chemical system by successively computing eq. 2.6 while systematically changing (optimizing) the positions of nuclei. This requires the Born-Oppenheimer approximation and leads to a potential energy surface. If the question is how a system changes over time, e.g., for the process of a precursor interacting with a substrate, MD approaches are applied. Here, the focus is on the dynamics of nuclei since the dynamic behavior of electrons (quantum dynamics) requires different theoretical approaches and is not relevant for (most parts) of AS-ALD. When the changes of a system over time are described with MD, Newton's equations of motion are typically used: