In the DeePMD-kit framework, a local coordinate frame should be constructed to preserve translational, rotational, and permutational (same atom exchange) symmetry. We set up the axes with the first axis along the direction to the nearest atom of the same ion type and the second axis along the direction to the nearest atom of the other ion type. Within this local coordinate system, the descriptors were assembled for each atom, including the full radial information for 20 atoms and the angular information for 40 atoms within the cutoff distance of R c = 6.8 Å.