Three scenarios, as shown in Figure , describe the interaction of two approaching H atoms in the lattice: For large distances, protiums do not interact with each other. If they approach each other, they would not interact in case of parallel spins (Figure , red curves, modeled as the triplet state), but spin relaxation will favor the much more stable singlet state. Independent if the protiums are bound to the same (Figure ) or neighboring (Figure ) layers, for distances above 2 Å, a proton-hydride pair with the hydride bound to a boron and proton bound to the nitrogen is the most stable configuration (black curves), and additional Coulomb attraction will readily form the most stable structure, namely H 2 molecule in the interstitial region of the h-BN lattice (blue triangles). Also if the H atoms are bound to the same layer, but at opposite interstitial sites, the formation of local proton-hydride pairs is energetically favored (Figure ). We now turn to the case of MoS 2 layered crystals, for which we performed the same type of WTMetaD simulations of H + and H inside the interstitials. Both protium and proton inside MoS 2 bind to S atoms with very similar binding strengths (see SI for details). The transport mechanism is very similar to that in h-BN: Initially, H + or H bind to sulfur atom S1 (see Figure ; black).