Dimensional classification and shape reliability in In_xSe_y clusters and 2D materials: a quasi-molecule perspective...

The reliable prediction of shapes of molecules and molecular materials from compact basis sets remains a key issue in electronic-structure theory. We examine here the dimensional classification - linear, planar or otherwise - of 2D In_xSe_y clusters and materials using subminimal-based density-functional-theory (DFT) within the quasi-molecule framework. By focusing on shape classes rather than numerical bond lengths and angles, the extremely compact STO-3G basis set is shown to preserve the correct dimensional character of chemical-bound systems. Optimization of small clusters with split-valence and polarized basis sets confirms that the overall structural topology so obtained is remarkably robust to basis-set truncation. The results, apparently reported here for the first time for 2D In₂Se₅, demonstrate that the essential features of bonding and geometry are already encoded in the valence-orbital symmetry of the quasi-molecular system. The analysis clarifies the conceptual limits of geometry as an emergent property of the Born-Oppenheimer surface and supports the rational use of subminimal bases for cluster explorations. Attention is paid to neutral molecules that can provide insights into crystalline lattices even when conventionally viewed as independent or unrelated components within the crystal structure. Possible implications of the popular periodic boundary conditions model are tentatively suggested. While of no apparent impact on the results reported here, the role of including relativistic effects has also been tested for some of the title systems.