Previously, our research group carried out molecular orbital studies of the interaction between MgATP and the myosin motor domain using the PM3 method of MOPAC97. In this study, we present recalculation results obtained using the PM6 method of MOPAC2009 and discuss the problems that may be encountered in further studies. Previous research carried out by our group in this field is introduced briefly. Furthermore, it is proposed that the increase in the distance between P_γ and the bridging O atom bound to P_β initiates the dissociation of γ-phosphate from ATP. Our results demonstrate that semi-empirical molecular orbital methods are useful in studying the chemical reactions underlying the initial step of ATP hydrolysis in the myosin motor domain.