## molecular dynamics

Molecular dynamics is a numerical method to compute the time evolution of a physical system by solving the classical Newtonian equations of motion of the set of N interacting atoms composing it (Fi = mai; i = 1,…,N; ai = d2ri/dt2), given an inter-atomic potential and starting from pre-defined initial conditions (constant volume, constant pressure…).

Molecular dynamics (MD) is a powerful, highly flexible technique used very widely to study innumerable physical problems in current materials science. It is irreplaceable whenever the knowledge of atomic-level detail is required in order to understand the phenomena of interest.

Inter-atomic potentials: inter-atomic potentials are analytical functions employed to determine the interaction forces between atoms as well as the energy of any configuration of atoms. They are the key elements governing the quality of numerical simulation at the atomic scale. The main applications of MD within PERFORM 60 can be summarised as follows:

- Simulations of Displacement Cascades: MD is the only method able to give an insight of the evolution of a displacement cascade inside a material as this phenomena is highly localized in space and evolve rapidly in time starting from a simple stochastic movement of atoms (few fs) due to the very high energy deposited by the incoming particle (neutron, proton, ion, electron…) inducing a thermal spike. The evacuation of this energy yields to a very disturbed element of volume the core of which consists on vacancies and vacancies clusters, interstitial clusters being at the periphery. Although MD is the most realistic method, simplified Binary Collision Approximation (BCA) can also be used to study the statistical aspect of the cascade characteristics. BCA is much faster than MD and allows to producing large database of primary damage, provided the simplified potential is properly fitted to reproduce the main aspects of MD results. MD is the most natural, reliable and complete technique for studying displacement cascades and therefore computing the source term of irradiation damage in materials. Since these phenomena cannot be directly observed in experiments, all the available information about cascades comes essentially from MD simulations
- Simulation of point-defects and point-defect clusters, in order to establish their stable configurations (formation energies, binding energies…), how they interact with each other (reaction mechanisms), and how they diffuse (migration energies and mechanisms), also in interplay with alloying elements.
- Simulation of extended defects, such as dislocations and grain boundaries, and their interaction with point-defects, point-defect clusters and different alloying chemical species