Low-Lying Isomers of Free-Space Halogen Clusters with Tetrahedral and Octahedral Symmetry in Relation to Stable Molecules Such as SF6

Motivated by previous work involving one of us (N.H.M.) on some 20 stable tetrahedral (t) and octahedral (o) molecules, including XF4 (X = C, Si, Ge), the natural orbital functional PNOF6 is here used to study the free-space halogen cluster t-F4. We consider an extended functional PNOF6­(N c) by coupling N c orbitals (N c > 1) to each orbital below the Fermi level, which improves the description of the electron pairs. Similar studies are presented for t-Cl4. The successful calculation on the stable molecule BrF5 (Theor. Chem. Acc. 2013, 132, 1298) has prompted a study of the clusters o-F6 and o-Cl6 too. The size relation with calculated known stable molecules and the experimental data are finally considered. In the case of the o-SF6, the geometry optimization with fixed octahedral symmetry has also been performed at the PNOF6(3) level of theory, leading to an equilibrium distance of 2.95 au in perfect agreement with the experiment. Our results confirm the multireferential character of Z4 and Z6 compounds (Z = H, F, Cl), in contrast with the single-reference character of the XZ4 (X = C, Si, Ge) and YZ6 (Y = S, Se, U) compounds; therefore, despite the clear patterns within a group, it is not possible to extrapolate the results to the case when the atomic number, X or Y, becomes zero.