The structure of smectic Q (SmQ) liquid crystal phase consisting of a dichiral molecule, called M7BBM7, was studied by submicrosecond molecular dynamics (MD) simulation. A detailed atomic model was used to study the stability of a model SmQ structure proposed by Levelut et al. (Levelut, A.-M.; Hallouin, E.; Bennemenn, D.; Heppke, G.; Lotzsch, D. J. Phys. II 1997, 7, 981) and its difference between (S,S)-, (S,R)-M7BBM7 and racemic mixture systems. Negative values of the fourth-rank orientational order parameter (< P-4 >), which characterize the model SmQ structure, were stably kept up to a 100 ns MD run only in the (S,S)-M7BBM7 system and lost in the other systems. The results correspond well to the marked chiral sensitivity in real systems where only the (S,S)-M7BBM7 system (among the three above-mentioned systems) shows the SmQ phase. Our simulation results imply that the asymmetric intramolecular potentials and resultant chirality-dependent molecular conformations are primarily responsible for keeping the negative values of < P-4 > and the model SmQ structure.