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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 ML_ABNCAR. Essentially the ML_ABCAR and the ML_ABNCAR files are the same and for continuation runs the ML_ABNCAR file is just copied to ML_ABCAR.
Here is a sample output how this file should look like: