Fig. shows cases of the comparison between ReaxFF and DFT for structures with W and H atoms. Fig. shows the atomic charges calculated by the EEM method in ReaxFF and Close-range interactions are crucial for simulating high-energy impact. However, DFT often yields erroneous results for small distances . Therefore, the close-range encounters for W-W and W-H atom pairs are trained on the ZBL potential , as shown in Fig. The trained energies encompass adsorption of H on a BCC (100) W surface (top, bridge, and hollow sites), interstitial H at the tetrahedral and octahedral sites, the energy difference between surface adsorbed H and subsurface H atom at the tetrahedral site, and binding of H to a tungsten vacancy site in the lattice. Å) for the reference curve is calculated using the ZBL potential, while PBE-DFT is used for distances > 0.6 Å. The two most stable WO3 lattices are orthorhombic and tetragonal, and Figs. 2d and 2-e show their equations of state (energy vs. volume relationship), respectively. The heats of formation (per atom) for the WO2, WO3 and W18O49 lattices are given in Fig. . Upon irradiation of W with O atoms, the formation of a WOx surface is likely, and the surface energy of the WO3 lattice is thus an important chemical space that we cover by training against the surface energies of the WO3 tetragonal and orthorhombic lattices along with the stable surfaces of W-BCC (Fig. ). In the event of sputtering, stable molecules of WOx are expected to form, and we train the formation energies of stable oligomers of tungsten oxides with respect to the WO3 molecule (Fig. ).