A full understanding of solid electrolytes requires consideration of their ion dynamics. The activation energy, E a , and Li diffusivity for each GB system and the bulk of each material was calculated and is shown in Figure . Also shown is the relative activation energies in the GBs compared to bulk, ∆E a = E GB a -E Bulk a , decomposed into components parallel and perpendicular to the GB plane. Aside from the antiperovskite Σ5{310} GBs, there is not a significant difference in the activation energies through and along the GB planes. In Li 3 OCl (Figure ), we find that the presence of GBs has a profound effect on Li-ion conductivity, with values of E a increased by around 0.2 eV in both GB systems, which is in good agreement with previous calculations that employed classical interatomic potentials. This provides further confirmation as to why computational predictions from simulations on bulk tend to overestimate the conductivity of antiperovskite materials compared to experiments which report activation energies as large as 0.6 eV, which is in better agreement with our GB simulations than with our bulk simulations. For Li 2 OHCl, the increases in activation energy compared to the bulk are far less pronounced (Figure ). In the case of Σ5{310} GB, the activation energy through the GB plane is actually slightly lower than the bulk, though it is offset by an increased activation energy along the GB plane. Again, the systems containing GBs are closer to experimental reports of activation energies in the range 0.5-0.6 eV. In comparison, for Li 3 PS 4 (Figure ) we find that the presence of GBs has a comparatively minor impact on ionic conduction, with activation energies for the bulk and the GBs ranging between 0.22 and 0.29 eV, which is in excellent agreement with experimentally reported values for crystalline β-Li 3 PS 4 of 0.24 eV and 0.27 eV. The lower impact of GBs in sulfides compared to oxides has been established previously using classical models and can be attributed to the stronger bonding between the cation and the oxide anion compared the sulfide anion. The behaviour of Li 3 InCl 6 GBs has not yet been reported, but we find that it is superior to the behaviour of Li 3 PS 4 (Figure ), with small changes to E a of no more than around 0.04 eV. Our predicted activation energies of 0.30-0.34 eV for the GB systems agrees remarkably well with reported experimental activation energies for Li 3 InCl 6 , which are in the range of around 0.32-0.35 eV. Overall, the interfacial diffusion behaviour of Li 3 InCl 6 shows an encouraging tolerance towards GBs. An obvious cause for the increase of E a at a GB would be perturbations to the electrostatic potential in the vicinity of the boundary. The segregation of charged defects may lead to the development of a space charge region and a corresponding electrostatic barrier, but approximating the space charge potential for a given material requires a detailed analysis of defect energetics and is beyond the scope of the current study. Nevertheless, aside from the space-charge region, electrostatic perturbations may arise as a property intrinsic to the GB structure. One method to quantify the effect of the electrostatic potential on an Li ion is to calculate the average electrostatic potential around the ion, which we shall call ϕ Li . The total electrostatic potential experiences large variations due to alternating layers of cations and anions, as well as larger perturbations in the voids of the GBs; focusing on ϕ Li allows us to investigate the effects that the GBs have specifically on the electrostatic environment of the Li ions. We calculated ϕ Li for a sample of 50 frames for each GB AIMD trajectory (800 K for Li 3 OCl and 600 K for Li 2 OHCl, Li 3 PS 4 and Li 3 InCl 6 ). This value is plotted as the mean of ϕ Li as a function of distance from the boundary (Figure ). In both of the Li 3 OCl GBs, the value of ϕ Li tends to vary significantly in the vicinity of the GBs, with an absolute difference from the bulk value of around 0.20 eV and 0.10 eV for the Σ3{112} and Σ5{310} GBs, respectively, which aligns fairly well with the observed changes in E a . In comparison, Li 2 OHCl shows a much weaker perturbation to ϕ Li , with Li 3 InCl 6 and Li 3 PS 4 showing a remarkably flat profile, suggesting that the considered GBs do not have a strong impact on the electrostatic landscape of Li, thereby providing an explanation for the small effect they have on conductivity. The antiperovskite Σ5{310} GBs both show higher variance in ϕ Li in the vicinity of the GB, which may explain the higher energetic barrier to conductivity along the GB compared to through the GB. The reasons for the stark differences in electrostatic behaviour can, in part, be attributed to the character of the anions. A larger anion will generally more polarizable than a smaller one, and therefore S and Cl will be able to more effectively screen electrostatic perturbations than O can. The OH anion is an interesting case as it differs from O by being a polyanion with a strong dipole which can undergo rotation and reorientation; such rotations would allow the anion to rotate to oppose electric fields created by electrostatic perturbations in the vicinity of the boundary. This reorientation of polyanions is known as the 'paddle-wheel effect', and it is known to also occur in Li 3 PS 4 . The degree of polyanion reorientation occurring in the system can be quantified by calculating the vector reorientation autocorrelation function, C(t), which is defined as