Effects of Langevin friction and time steps in the molecular dynamics simulation of nanoindentation

In the simplest form of molecular dynamics (MD), the system evolves according to Newton's equations of motion with its total energy conserved. Moreover, to model systems at constant temperatures, a modified set of equations of motion, called a thermostat, can be used. Langevin thermostat with one parameter called the Langevin friction coefficient is a popular thermostat, but problems may arise when it is applied to nonequilibrium systems without tuning the friction coefficient. Additionally, the size of MD time step also plays a pivotal role in determining accuracy and efficiency. In this study, we consider the effects of time step and Langevin friction on MD simulations. First, we evaluate the effectiveness of the Langevin thermostat for different combinations of time step and friction coefficient. Second, nanoindentation of Nickel crystals are carried out for five different combinations of Langevin friction and time steps. The simulation results reveal noticeable differences in mechanical properties as the dynamics changes from a constant energy to constant temperature simulations with increasing friction coefficients. The structural characteristics are also quantified using the common neighbour analysis, centrosymmetry parameter and dislocation analysis, all of which show a similar trend of more atoms being deformed with increasing friction.