title: Low-Lying Isomers of Free-Space Halogen Clusters with Tetrahedral and Octahedral Symmetry in Relation to Stable Molecules Such as SF6.
abstract: Motivated by previous work involving one of us (N.H.M.) on some 20 stable tetrahedral (t) and octahedral (o) molecules, including XF(4) (X = C, Si, Ge), the natural orbital functional PNOF6 is here used to study the free-space halogen cluster t-F(4). We consider an extended functional PNOF6(Nc) by coupling Nc orbitals (Nc > 1) to each orbital below the Fermi level, which improves the description of the electron pairs. Similar studies are presented for t-Cl(4). The successful calculation on the stable molecule BrF(5) (Theor. Chem. Acc. 2013, 132, 1298) has prompted a study of the clusters o-F(6) and o-Cl(6) too. The size relation with calculated known stable molecules and the experimental data are finally considered. In the case of the o-SF(6), 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 Z(4) and Z(6) compounds (Z = H, F, Cl), in contrast with the single-reference character of the XZ(4) (X = C, Si, Ge) and YZ(6) (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.
authors.affiliation: Donostia International Physics Center (DIPC) , Paseo Manuel de Lardizabal, 4, 20018 Donostia, Euskadi, Spain.|Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) , Manuel Lardiazabal Pasealekua, 3, 20018 Donostia, Euskadi, Spain.|IKERBASQUE, Basque Foundation for Science , Maria Diaz de Haro 3, 48011 Bilbao, Euskadi, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Piris
authors.affiliation: Donostia International Physics Center (DIPC) , Paseo Manuel de Lardizabal, 4, 20018 Donostia, Euskadi, Spain.|Department of Physics, University of Antwerp , Groenenborgerlaan 171, 2020 Antwerp, Belgium.|Department of Physics, Oxford University , Parks Road, OX1 3PU Oxford, England.
authors.forename: N H
authors.initials: NH
authors.lastname: March