Static and dynamic structure of monomers, dimers and trimers of HgCl2 from density-functional calculations.

We report relativistic density-functional calculations for the equilibrium structures and the vi- brational frequencies of the (HgCl2 )n molecules with n = 1 to 3, as part of a broad exploration of the potential energy landscape of these compounds that will later be used to develop their pseudoclassical inter- atomic force laws. The calculations are carried out both in a physical plane-waves-expansion approach and in a quantum-chemical localized-Gaussians-expansion approach, with mutually consistent results within their expected accuracy, and are supplemented by analysis of the bond type and of the valence-electrons localization. The relativistic results are also compared with those of analogous non-relativistic calcula- tions. For the monomer and the dimer we find close agreement with the earlier results of Kaupp and von Schnering and of Donald, Hargittai and Hoffmann, and in particular for the mechanical-equilibrium shape of the dimer we confirm their prediction of a major symmetry-breaking distortion driven by rela- tivistic effects. We find an analogous relativistic structural distortion for the trimer, leading to alternative mechanical-equilibrium shapes that can all be viewed as resulting from the direct addition of a monomer to a dimer. The basic ground-state structures of the trimer clearly are precursors to the unique crystal structure of HgCl2 as a lamellar crystal formed from stripes of Cl-Hg-Cl molecules.