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A recent experiment has reported the first observation of a zero-field fractional Chern insulator (FCI) phase in twisted bilayer moiré superlattices [J. Cai , Signatures of fractional quantum anomalous Hall states in twisted , Nature (London) 622, 63 (2023).NATUAS0028-083610.1038/s41586-023-06289-w]. The experimental observation is at an unexpected large twist angle 3.7° and calls for a better understanding of the FCI in real materials. In this Letter, we perform large-scale density functional theory calculation for the twisted bilayer and find that lattice reconstruction is crucial for the appearance of an isolated flat Chern band. The existence of the FCI state at is confirmed by exact diagonalization. We establish phase diagrams with respect to the twist angle and electron interaction, which reveal an optimal twist angle of 3.5° for the observation of FCI. We further demonstrate that an external …