The structural information of Si(001)-C2H2 chemisorbed surface has been presented on the basis of the first-principles total-energy calculations. The curves of grand canonical potentials of various surface coverages as the function of C2H2 chemical potential pass through the almost identical point, which indicates small interactions between admolecules. Our results suggest that the effect of the degrees of freedom of remaining surface dimers is important to determine the optimal adsorption structures. In addition, the dimer cleaved model is found to be a metastable state only with a coverage of theta = 1, and it is not realized under the thermal equilibrium condition based on grand canonical potential calculations.