Where the force F acting is given as the mass of the system m multiplied by the acceleration (derivative of the velocity v w.r.t. time t). The approach now is to define starting conditions with a specific arrangement of atoms and assign velocities to these atoms in order to compute the forces acting on the atoms. The forces after a certain timestep are then computed with different algorithms. The choice of the timestep (typically in the order of the fastest molecular vibration in the system, ca. 0.5-1 fs) and the choice of the algorithm (velocity verlet, leapfrog, etc.) is important for the accuracy of the resulting MD. When this approach is applied many consecutive times, the result is a MD trajectory which describes the behavior of the chemical system over the simulated time (in the order of picoseconds for AIMD). Further crucial choices for the MD algorithm are the ensemble (the decision which state functions are kept constant: pressure p, temperature T, number of particles N, volume V, energy E) and the thermostat (Nosé-Hoover, Anderson, etc.) which helps to keep these values constant. There are many details to be considered when actually carrying out an MD computation which are beyond the scope of this short description and can be found in the literature. What is crucial for AS-ALD modelling is the question of how to treat the electrons in this MD approach. There are currently three major approaches to describing intermolecular interactions (the potential).