*2.5. Complexity Calculations*

In order to characterize and quantify the impact of each substructural units on the formation of a particular architecture, the structural complexity approach recently developed by S.V. Krivovichev [19–23], which allows comparison of the structures in terms of their information content, was used.

The complexity of the crystal structure was estimated as a Shannon information content per atom (*IG*) and per unit cell (*IG,total*) using the following equations:

$$I\_{\mathcal{G}} = -\sum\_{i=1}^{k} p\_i \log\_2 p\_i \tag{bits/atom} \tag{1}$$

$$I\_{\rm G,total} = -\upsilon \, I\_{\rm G} = -\upsilon \sum\_{i=1}^{k} p\_i \, \log\_2 \, p\_i \quad (\text{bits/cell}), \tag{2}$$

where *k* is the number of different crystallographic orbits (independent sites) in the structure and *pi* is the random choice probability for an atom from the *i-*th crystallographic orbit, that is:

$$p\_i = m\_i / \upsilon\_\prime \tag{3}$$

where *mi* is a multiplicity of a crystallographic orbit (i.e., the number of atoms of a specific Wyckoff site in the reduced unit cell), and *v* is the total number of atoms in the reduced unit cell.

The reliable correlation of structural complexity parameters is possible only for compounds with the same or very close chemical composition (e.g., polymorphs), whereas changes in the hydration state, nature of interstitial complexes, and size and shape of organic molecules could significantly affect the overall complexity behavior. In this light, within the current crystal chemical review, structural complexity parameters of various building blocks (uranyl selenite units, interstitial structure, H-bonding system) were calculated to analyze their contributions to the complexity of the whole structure. This approach suggested by S.V. Krivovichev [24] and recently successfully implemented in [25,26] allows the factors that influence the symmetry preservation or reduction of uranyl selenite units to be revealed, and it shows which of the multiple blocks plays the most important role in a particular structure formation.
