or S3 in the adjacent layer. The next transfer can then either go back to the starting point, or hop to the next site, and so on. The trajectories are very similar in both cases, therefore, in Figure , we only show the example of H + . Thus, both H and H + in MoS 2 are also transported along a zigzag path, hopping between the neighboring layers and the transfer is supported by low-energy shear modes. The corresponding FES plots are given in Figure and. The FES profiles show the free energy barriers of 0.11 eV for H + and 0.09 eV for H atom. Unlike in the case of h-BN, the barriers in MoS 2 are very similar, because of the same type of bonding atoms. These energy barriers are smaller than in the case of h-BN for the proton, while slightly higher for H atom inside h-BN. However, in contrast to graphite or h-BN, defects are much more common in the natural or synthesized MoS 2 , typically with a dominant occurrence of sulfur vacancies. Therefore, we studied also the diffusion of H species in the presence of this defect type. The WTMetaD results are shown in Figure , revealing that both H and H + are attracted strongly by the S atoms surrounding the vacancy site. The vacancy traps the travelling species and this effect appears to be stronger for H atoms than for protons. The FES for protons is 0.10 eV, close to the barriers in a perfect material. For H atom, we have obtained two different barriers, 0.06 and 0.25 eV, which might be due to the charge redistribution close to the vacancy and different binding strengths to the S atoms in the vicinity of the vacancy and far from it (see Figure ). The smaller barrier corresponds to the hopping between binding sites far from the vacancy, which is similar to the case of a perfect system. The higher barrier corresponds to the hopping between binding sites close to the vacancy, showing that these sites interact much stronger with H atoms. The vacancies, therefore, suppress the overall transport properties of MoS 2 , which could explain why in the experiment h-BN is a better hydrogen conducting material than MoS 2 . Finally, we have investigated H atom inside graphite (proton gets immediately neutralized inside this system). Here, however, we have not observed any H atom transfers with free energy barriers similar to the other materials, meaning that this process would require much higher activation energies and would be improbable in the experimental conditions (see Figure ).