Different wall time for the same simulation timescale

GROMACS version: 2019.1
GROMACS modification: No
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I ran two simulations (back and forth) with the following differences in md.mdp file

  1. dt = 1 fs, number of steps = 5* 10^8, nstxout = 5000 (every 5 ps)
  2. dt = 0.5 fs, number of steps = 10 * 10^8, nstxout = 200000 (every 100 ps)
    The total md simulation time is 500 ns in both cases, however the computer wall time is 0.006 hr/ns for the first one and 0.011 hr/ns for the second one. I don’t understand why this happens.

If your time step is cut in half, it takes twice as many steps to simulate a given amount of time. So it takes twice as long.