In a molecular dynamics calculation of a monatomic system, the average kinetic energy per atom was 0.0517 eV. Which is closest to the temperature of the system obtained from the equipartition theorem? Take the Boltzmann constant as k = 8.617×10⁻⁵ eV/K.

Answer: A. About 400 K

Each atom has three translational degrees of freedom, each carrying kinetic energy kT/2, so the average kinetic energy is (3/2)kT. Thus T = 2 × 0.0517/(3 × 8.617×10⁻⁵) ≈ 400 K. About 600 K takes the kinetic energy as kT, and about 200 K takes it as 3kT. In molecular dynamics the instantaneous temperature is found from the atomic velocities in this way, and when the temperature is to be held constant, the velocities are adjusted by coupling to a heat bath (a thermostat). With few atoms the instantaneous temperature fluctuates strongly, so it is evaluated as a time average. In small systems the value changes slightly with how the degrees of freedom are counted, for example by removing the center-of-mass motion.