We carry out a comparative study of the energetics and dynamics of Si-Si, G
e-Ge, and Ge-Si ad-dimers on top of a dimer row in the Si(001) surface, usi
ng first-principles calculations. The dynamic appearance of a Ge-Si dimer i
s distinctively different from that of a Si-Si or Ge-Ge dimer, providing a
unique way for its identification by scanning tunneling microscopy (STM). I
ts "rocking" motion, observed in STM, actually reflects a 180 degrees rotat
ion of the dimer, involving a piecewise-rotation mechanism. The calculated
energy barrier of 0.74 eV is in good agreement with the experimental value
of 0.82 eV.