Potential energy

  The average potential energy V is obtained by averaging its instantaneous value, which is usually obtained straightforwardly at the same time as the force computation is made. For instance, in the case of two-body interactions

$$V(t) = \sum_i \sum_{j\gt i} \phi\left( \vert {\bf r}_i(t) - {\bf r}_j(t) \vert \right)$$

Even if not strictly necessary to perform the time integration (forces are all is needed), knowledge of V is required to verify energy conservation. This is an important check to do in any MD simulation.