Hybrid quantum/molecular mechanics models (QM/MM methods) are widely used in material and molecular simulations when MM models do not provide sufficient accuracy but pure QM models are computationally prohibitive. Adaptive QM/MM coupling methods feature on-the-fly classification of atoms during the simulation, allowing the QM and MM subsystems to be updated as needed. We propose such an adaptive QM/MM method for material defect simulations based on a new residual based a posteriori error estimator, which provides both lower and upper bounds for the true error. We compare this new estimator with that of the Atomistic/Continuum (A/C) coupling methods. Based on this new estimator, we propose an inner-outer adaptive strategy, which includes an outer adaptive algorithm to select the QM and MM regions, and an inner adaptive algorithm to compute the approximate estimator for the selection up to a given accuracy. We validate the analysis and illustrate the effectiveness of the new scheme on numerical simulations for material defects.