1. convert the POSCAR to a pymatgen Structure object using Structure.from_file() method in pymatgen
2. Apply any featurizer directly to that Structure object
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from matminer.featurizers.composition import ElementProperty
from matminer.featurizers.structure import DensityFeatures
from pymatgen import Structure
s = Structure.from_file("/Users/ajain/Desktop/POSCAR")
comp_features = ElementProperty.from_preset("magpie").featurize(s.composition)
print(comp_features)
s_features = DensityFeatures().featurize(s)
print(s_features)
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If you have a lot of structures (many POSCARs), you could also first create a DataFrame with a "structure" column and then follow the other tutorials as needed