Problematically for those working in the field of MD simulation, this assumption only holds true for timescales that are on the order of a Poincaré recurrence time, which is longer than the age of the universe. Therefore, because for any realistically obtainable single trajectory of a dynamical system ρ t does not asymptotically approach ρ eq , the value of a given observable obtained from an individual molecular dynamics trajectory cannot be equated to the true value of the observable that would arise if phase space were ergodically sampled.