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A Velocity of the center of mass for MD simulations

  1. Dec 28, 2017 #1
    When calculating the center of mass for an MD simulation, I've seen a few codes that subtract the average velocity from each atoms velocity. I think the reasoning behind this is that you want the velocity of center of mass for the simulation to be zero, which I understand, but I don't understand computationally how subtracting the average velocity from each velocity achieves this.
  2. jcsd
  3. Dec 28, 2017 #2


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    Can you see that the [mass-weighted] average velocity is equal to the velocity of the center of mass?
  4. Dec 28, 2017 #3
    I don't see that. This is how I'm seeing it:

    mass-weighted average velocity = (sum each atom's momentum) / (num atoms)

    velocity center of mass = (sum of each atom's momentum) / (total mass)

    And if they are equal, how does help me understand my original question?
  5. Dec 29, 2017 #4


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    The correct way to compute a weighted average is to divide by the sum of the weights, not the count of items.
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