def draw_forces(ax, pos, vel, **kwargs):
  """ draw forces on atoms

  Args:
   ax (plt.Axes): matplotlib Axes object, must have projection='3d'
   pos (np.array): array of atomic positions
   vel (np.array): array of forces on each atom (or velocities)
   kwargs (dict,optional): keyword arguments passed to plt.plot
  Returns:
   list: a list of plt.Line3D
  """
  x, y, z = zip(*pos)
  vx, vy, vz = zip(*vel)
  qvs = ax.quiver(x, y, z, vx, vy, vz, **kwargs)
  return qvs