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This file is used within the machine learning force field method. It contains the ab initio data (positions, energies, forces and stress tensors) from previous calculations. It is used for continuation runs (ML_FF_ISTART=1 or ML_FF_ISTART=2). The updated data is written to ABNCAR. Essentially the ABCAR and the ABNCAR files are the same and for continuation runs the ABNCAR file is just copied to ABCAR.