*2.1. Geometry Optimization–Calixarenes*

The first step in the analysis of complexes is the selection of the most suitable conformers for the calixarene host. In order to facilitate this task, we performed a query in the Cambridge Structural Database (CSD) [78] for accessible calix[6]arene structures. This search resulted in three selected conformers: pinched-cone (CSD-VARGUR [79]) – found as the most stable in nonhydrogen-bonding solvents [80], 1,2,3-alternate (CSD-REGWIM [81]) – preferred in hydrogen-bonding solvents [80], and recently crystalized winged-cone one (CSD-REGWEI [81]). Starting geometries of these conformations were extracted from the corresponding crystal structures, and several starting geometries were generated with the Tinker program [82] by freezing all but dihedral angles of the hydroxyl groups. Then, the geometry optimization was performed with Density Functional Theory (DFT) using the B97-D3 functional [83], which is a popular choice for organic molecules. Electron repulsion integrals were treated within the Density Fitting (DF) approximation [84] and the def2-TZVP orbital basis set [85] was used with the corresponding default auxiliary DF basis. The resulting geometries are similar to the starting ones (no hydroxy group has switched from *u* to *d* or *vice versa* during the optimization), so the final conformers will be denoted as *pc* (from pinched), *al* (from alternate), and *wc* (from winged-cone), as the original structures from the CSD. The stability of these conformers for the CX case was tested by a modification of their geometries through a rotation of selected phenyl rings and reoptimization, which either led to the original conformer, or to local minima energetically higher than *pc*, *al*, or *wc*.
