Pymatgen

This is the main computational materials science library in Python, powering the Materials Project. You use it to manipulate crystal structures, convert between 100+ file formats (CIF, POSCAR, XYZ), analyze symmetry and space groups, compute phase diagrams, and process electronic structure data like band structures and DOS. It integrates directly with Materials Project's database and handles output from VASP, Gaussian, and Quantum ESPRESSO. The API is straightforward: Structure.from_file() reads anything, structure.to() writes to any format, and SpacegroupAnalyzer gives you symmetry info. If you're doing DFT calculations or working with crystallographic data, you're probably already using this or you should be.