## 4 Conclusions and Outlook  
A first-principles DFT study is presented to investigate the Si-rich zeolite clinoptilolite within CP2K code based on a B97-D3 functional. We propose a different methodology that makes calculations computationally tractable for complex large systems such as clinoptilolites. In addition, our procedure can provide a full guide, or at least a good starting point, for conducting accurate DFT calculations on other microporous zeolitic systems, such as heulandites, by changing the relative Al occupancy of the different T sites. Extensive convergence tests on all silica models were performed to find a robust set of input parameters and then Al atoms were incorporated via substitution, for which hundreds of models were generated. Moreover, non-Lowensteinian configurations were excluded. A comparison of different density functionals was carried out for models dominated by Na as a compensating cation, whereby B97-D3 showed a robust computational performance and produced bond lengths that were consistent with the experimental findings. We explored Al and cation distributions, and the findings highlight the association between aluminum atoms and extra-framework compensating cations; the movement of the extraframework content depends on the Al distribution and the long-range cation-cation repulsion. For our present case, involving compositions containing six Al atoms,the presence of Al at the T2 and T3 sites facilitates the coordination of the compensating extra-framework Na or Ca with the framework oxygens, while Ca does not avoid Al at T1 sites as Ca is situated in channel B. The presence of Ca in channel A is a source of considerable instability and it shows a relatively different behavior that depends on several factors. Ca prefers channel B and therefore has more stable interactions with the framework content. Furthermore, the movement of those extra-framework cations and their interaction and/or coordination with the framework are crucial for stabilizing the energy of the entire structure. The results also suggest that in Na-rich compositions with six Al atoms, Na atoms prefer to increase their coordination with the oxygen atoms surrounding the Al3 atoms more than those surrounding the Al2 atoms. Such a trend, however, was not observed with Ca. The latter will also not avoid Al at T5 sites, in contrast with Na. K, on the other hand, prefers Al occupation at the T1 and T3 sites. It also favors being present in channel C. An optimization of the basis sets was carried out and the addition of more orbitals for Ca cations was shown to influence Ca movement within the cell and thus affected how it interacts and coordinates with the framework. Therefore, comparing the behavior of zeolites before and after optimization of the basis sets is worth looking into in a future follow-up work. Speaking of which, the following work will introduce the following:  
1. Surface models of the different clinoptilolite models studied here. 2. An exploration of the interaction of the emerging contaminants, such as PPCPS, with those surface models. With the increasing complexity of electronic structure codes, researchers often default to preset values for their input variables, without investigating how this can lead to calculations that are far from accurate. In this work, we attempt to show that when DFT is used properly, it can provide reliable conclusions for many applications of interest. This requires that researchers opt for the most suitable input variables for the system at hand: plane wave cutoff energy, good convergence protocols, and, last but not least, exchange-correlation functionals. Most of these parameters depend on the material systems as well as on benchmarking and experience; however, in the end, we all need to remind ourselves of the following [60; 61]:  
_'We must now note a confusing, but important, notion that while DFT is exact in principle, in practice it is implemented with approximations.