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This binary file contains a knewly created force field from machine learning runs with the options ML_MODE=train, refit or select. It's structure is identical to the ML_FF file. To be able to use the new force field from ML_FFN file it has to be simply copied to ML_FF and the INCAR tag ML_MODE=run has to be set.

Since VASP 6.4.0 the ML_FFN file starts with an ASCII header containing the most important INCAR tags in effect during generation of this force field. In Linux shells this can be easily extracted issuing the following command:

head -n 1 ML_FFN

The output may look like this:

ML_FF 0.2.1 binary { "date" : "2023-03-16T13:49:44.829", "ML_LFAST" : False, "ML_DESC_TYPE" :   0, "types" : [ "Si" ], "training_structures" : 984, "local_reference_cfgs" : [ 110 ], "descriptors" : [ 142 ], "ML_IALGO_LINREG" : 3, "ML_RCUT1" :  6.0000E+00, "ML_RCUT2" :  6.0000E+00, "ML_W1" :  1.0000E-01, "ML_SION1" :  5.0000E-01, "ML_SION2" :  5.0000E-01, "ML_LMAX2" : 4, "ML_MRB1" : 8, "ML_MRB2" : 8, "ML_IWEIGHT" : 3, "ML_WTOTEN" :  1.0000E+00, "ML_WTIFOR" :  1.0000E+00, "ML_WTSIF" :  1.0000E-10 }

followed by some extra spaces (because the header is always 4096 characters long). The timestamp following "date" is also written to the FFOUT lines in ML_LOGFILE. The "ML_LFAST" item allows you to check whether this force field is ready for fast prediction mode.