The behavior of the energy of stacking fault defects in copper as a function of external strain and temperature is investigated making use of molecular-dynamics simulations. Atomic interactions are modeled by an effective-medium theory potential. Intrinsic, extrinsic, and-twinning faults are considered. Our results suggest that the stability of stacking-fault defects in copper increases with temperature and decreases with applied compressive strain. In addition, we point out some difficulties posed by the application of finite range model potentials to the study of low-energy defects. To show that-these difficulties are quite general in nature we also compute the stacking-fault energy (SFE) from an embedded atom model potential. Our results indicate that the SFE computed from model potentials displays a spurious change of sign with increasing compressive strain. [S0163-1829(99)02245-6].