The first milestone of ReaxFF parameter training for the Mg/Mg metal interactions was to reproduce the bulk properties of Mg. Therefore, lattice constants, cohesive energy values, and bulk moduli of the crystal phases hcp, fcc, bcc, a15, and sc were introduced to the training set. Table compares the DFT training data and the results obtained with the fitted ReaxFF force field. We only trained bonding but excluded off-diagonal, valence, or torsion parameters for the Mg/Mg interactions, which means that first, the a/c ratio always corresponds with (8/3) 1/2 to the ideal value of a closed-packed crystal, and second, the hcp and fcc cohesive energy will be identical. Experimentally measured lattice constants 63 of the hcp crystal phase are accurately reproduced by ReaxFF (a 0 : both 3.18 Å, c 0 : 5.20 Å vs. 5.21 Å), which is also true for the cohesive energy (both 1.50 eV) and bulk modulus (38.5 GPa vs. 36.5 GPa). The cohesive energy, in particular, is an important measure to describe bond strengths, and was therefore weighted more strongly during the training process; it varies at most by 0.01 eV for all crystal phases. This is also reflected in the relative stability between the crystal phases, which are in good agreement. ReaxFF slightly overestimates the bulk modulus for all crystal phases. This behavior is illustrated in the equation of state (EOS) curves presented in Figure and is a result of an increase of the ReaxFF energy values at higher volumes. is the most stable Mg surface, followed by Mg(1010)A and Mg(1011). This trend is reproduced by the trained ReaxFF force field as illustrated in Further, the trained ReaxFF force field well reproduces the calculated surface fractions in the Wulff construction (Table ). Mg(1011) has the largest area percentage (56.9 % vs.