The recent revolutionary increase of power conversion efficiency demonstrated by perovskite-based solar cells has given rise to great interest from the scientific community. In the last few years, an ever-increasing number of experimental and theoretical studies have been performed on 3-dimensional hybrid halide perovskite such as CH3NH3PbX3 (X=Br, I). Here, we consider CH3NH3PbBr3 that is known to have good luminescent properties. So far, the surface defects are one of the major factors for fluorescence quenching [1]. It has recently been reported that exposition to moisture and O2 gas can recover the luminescent properties [2-4]. In order to elucidate the luminescence quenching-reactivation mechanism, we herein carefully inspect the structural, electronic and optical properties both for bulk and (001) surface of CH3NH3PbBr3 using atomic scale simulations. References: [1] G. Grancini et al. Chem. Sci. (2015), 6, 7305-7310; [2] H.-H Fang et al. Sci. Adv. (2016), 2, 7; [3] H. Wei et al. Nature Photonics (2016), 10, 333; [4] Yin, W.-J et al. Adv. Electron. Mater. (2015), 1, 1500044.