## 3 Results and Discussion
### Migration of Ca Ca Cations  
Analyzing the 84 configurations of our models, one can deduce the following. While Na tends to favor the presence of Al at T2 and T3 sites and avoids Al at T1 sites, Ca does not necessarily avoid Al at T1 sites, especially if Ca is present in channel B. This situation becomes intricate when Ca is positioned in channel A: in some instances, Al1 was favored, approached only marginally, or entirely avoided. For instance, Al1 is favored if Al2 is also present in the 10 MR. This is evident in the configuration shown in Figure 7, which is the most stable configuration. Ca approached the sides of the 10 MR of channel A and was coordinated with O1, two O2s, and O3, whereby the distance between Ca and Al1 was -5.35 A rather than the \(\sim\)5.45 A observed before relaxation. On the other hand, in Figure 8a, Ca increased its distance from Al1 to \(\sim\)5.59 A instead of \(\sim\)5.45 A when Al3 was present. The configuration depicted in Figure 8b is an exception to this pattern; not only did Ca avoid Al1, it also avoided Al3, which was the only instance in which Ca distanced itself from Al3. Ca increased its distance from \(\sim\)3.56 A to \(\sim\)3.57 A with Al3 and from \(\sim\)5.45 A to \(\sim\)5.58 A with Al1, and both Ca atoms in both channels moved toward the intersection between channels A and C. Moreover, Al1 was generally avoided if there was no Al2 or Al3 in the 10 MR of channel A, as shown in Figure 8c, and was one of the highest-energy configurations. Ca increased its distance to Al1 from \(\sim\)5.46 A to \(\sim\)5.47 A and did not coordinate with O1 in this instance, meaning it was coordinated three times, unlike most of the configurations, in which it was coordinated four times. In Ca-clinoptilolites, a contraction was observed specifically in lattice parameter \(b\) but not in lattice parameters \(a\) or \(c\), which were well reproduced, as reported elsewhere [11]. Experimentally, variations in cell dimensions are to be expected because of dehydration or ion exchange for example. It could also be the strong coordination and/or interaction of the compensating extra-framework Ca atoms with the framework that leads to this contraction. A similar scenario was noted in Na-rich clinoptilolites, albeit less prominently. Na's behavior remained consistent across all configurations, exhibiting a straightforward pattern. However, the behavior of Ca was more complex and contingent upon its placement in either channel A or channel B. While Ca generally did not avoid Al1 in most configurations, it showed a preference for Al at T2 and T3 sites over T1 when situated in channel A. Interestingly, we speculate that Ca will be required if Al occupies T5, a finding supported by previous research [11]. This suggests, for our case with dehydrated models, that Ca could be a more favorable cation in clinoptilolite compared to Na, and possibly even the most favorable of all considered cations. To validate this conclusion, further investigations with other cations are necessary. However, it is important to note that the interaction between Ca, as an extraframework cation, and the framework content is intricate and influenced by various factors. This complexity likely contributed to the significant changes  
Figure 7: Schematic showing the most stable configuration of Ca-clinoptilolites among the 84 models, shown in the c-direction, with (**a**) portraying the atomic shifts in the extra-framework cations post-optimization and (**b**) the sites of extra-framework cations prior to optimization. observed in the convergence behavior of the SCF cycle when Ca was introduced into our system. It is worth reiterating that the primary goal of this study is to identify and develop various clinoptilolite models with different cation arrangements. These models will serve as the basis for the generation of surface models in future work, where we will focus on analyzing adsorption at the surfaces rather than within the bulk structures.