.. index:: Tutorials; time_convergence

Time convergence
****************
Here, a short demonstration on how time-correlation typically converges with total simulation time is provided for FCC Al at 300 K.
To get a converged correlation function, e.g., in order to extract a phonon lifetime, one requires a simulation that is on the order of 100 to 1000 times longer than the lifetime itself.
Rather than running a single long MD simulation, one can also run many shorter simulations and average their correlation functions.

In the figure below, the decay time of the correlation function is about 1 to 2 ps.
The :math:`C_L(q, t)` is pretty well-converged after 200 ps and very well converged after 2000 ps.


.. figure:: figs/Al_solid_acf_convergence.svg
    :align: center
    :width: 75%

    Current correlation :math:`C_L(q, t)` for simulations of varying lengths.
