The genetic algorithm is combined with the density functional theory to predict how the cylindrical spatial confinement affects the structural characteristics and optical adsorption spectra of the low‐energy Si18H12 and Si19H12 isomers. Retrieved ground states (minimum energy states) of Si18H12 and Si19H12 isomers significantly differ from the earlier proposed “ultrastable” aromatic molecular systems and prismanes. According to our calculations, they are represented by the almost spherical endohedral buckyballs. In contrast to pure silicic clusters (Si18 or Si19), the most of Si atoms in the low‐energy Si18H12 and Si19H12 are four‐coordinated. Spatial confinement results in more oblong structures with the different optical spectra. Prismanes under confinement get some energy advantages over the spherical structures, but despite this they do not become the most energetically favorable ones. Thus, the current results warrant however further research on the spatial confinement of silicic prismanes, as our study suggests challenges in devising the method of their synthesis without additional chemical techniques.