Two kinds of interaction model are proposed to the asymmetric dimer mo
del of the Si(100) reconstructed surface. One is the short range inter
action model, and the other is the one including the longer range elec
tric dipolar interaction. Short range order structures at step edges a
re examined for the two kinds of interaction model, and some differenc
es are found. An attempt is made to compare the results with the STM i
mages.