## 3 Results and Discussion
### Na-Ca Clinoptilolite  
Most of the issues encountered during the optimization of structures dominated by Ca were resolved when Na was introduced alongside Ca, providing two atoms of each. First, this effectively addressed the local charge imbalances surrounding Al atoms, which, in many configurations, were not adequately satisfied with only three Ca atoms distributed unevenly throughout the structure, leading to significant instabilities. Another factor contributing to this is the stability observed when Ca was absent from channel A, being exclusively present in channel B while Na occupied channel A. This arrangement resulted in all cavities being occupied with cations or more cations surrounding the tetrahedra with Al atoms, thus leading to more stable configurations, highlighting the stabilizing impact of the extra-framework components on the overall structure.  
The distances between Na and Al at T2 and T3 sites decreased, while they increased with Al at T1 sites following full structural relaxations. In contrast, Ca showed stronger interaction with the framework when Al was present at any T site. Moreover, the discrepancies between the cationic distances with the framework oxygens and the experimental measurements that were mentioned earlier were reduced, particularly for cations located in the larger channel A. Our observations also suggest that configurations with three Al atoms distributed on each side of the unit cell along the c-direction tend to be more stable than those with a 4:2 ratio on both sides of the unit cell. The latter scenario will lead to an unstable configuration and/or serious convergence issues in the SCF cycle.  
Ca behavior significantly depends on the position of Al atoms. For example, Figure 9 illustrates various sections of several structures where Ca shifts in response to the positions of the Al atoms, a behavior that differs somewhat from that of Na cations. Na is relatively more conserved in the unit cell; in other words, it moves within fewer angstroms whereas Ca has the ability to traverse from the left side to the right end of the channel. On the same note, in Figure 9a, the presence of Al2 and Al1 on opposite sides of the structure led to a change in Ca position at the left side of the 8 MR of channel B to a position near the center of the channel to maximize its coordination with the framework oxygen. However, its proximity to Al1 was more pronounced due to Al2 being surrounded by Na1. Likewise, in Figure 9b, Ca shifted further toward the middle of the channel, where the presence of Al in the structure is more pronounced, and extended its coordination with the framework oxygens. When it was not surrounded by any Al on the right side of the channel, as shown in Figure 10c, it relocated to the opposite side, where Al3, Al2, and Al1 were situated--a behavior that one will never experience with Na cations, which tend to move conservatively within the channels of clinoptilolites. In the last figure, Ca2 was surrounded by Al1 from the bottom left and Al2 from the bottom right side of the channel; this led to Ca moving to the bottom of the channel, positioning itself more towards the left side, as the Al1 atom lacked any other cation counterpart, unlike the Al2 atom on the opposite side.  
It is worth reiterating that a minor contraction in the lattice parameter \(b\) was observed in Na-clinoptilolite, whereas in Ca-clinoptilolite, the contraction in the \(b\) lattice parameter was more pronounced. However, this was not the case when Na and Ca were added together to our models. As indicated in the lattice parameters provided in the Supplementary Material, the \(a\), \(b\), and \(c\) lattice parameters were very well reproduced in those latter structures.