Structure, bonding, relativistic effects, and dispersion in the group 12 dihalide (MX) clusters, with lessons from the extended solids

The prediction of crystal structures for inorganic solids remains a challenge at the frontier of materials science. We examine the relationship between the group 12 dihalide monomers, small oligomers (in particular the trimers), and the extended solids. The clusters are investigated at density functional B3PW91 and MP2 computational levels and at the M06-2X, B97D3, and CCSD(T) levels for the first time for special cases where dispersion is important. Significant patterns in the bonding preferences in the dimers and extended solids of these systems are known to reflect the rigidity of the linear monomers. The trimers evolve typically as one-dimensional extensions of the dimers, but the HgX systems are different: They form loosely bound clusters with a few competitive geometries emerging that imply several potential directions for continued and more strongly bond higher-order clusters. The very weak intermolecular interactions in the Hg systems appear only when relativistic effects are turned on. Otherwise, they behave precisely like their Zn and Cd analogues. The HgF trimers, which have not been considered before, are examined in detail as well. These trimers - especially the fluorides - are somewhat more polar than the isolated monomers and dimers, and we find evidence for a gradual switch to the ionic bonding regime in the HgF clusters on the way to the fluorite extended solid structure. Differences between geometrical preferences obtained at the MP2 versus the B3PW91 levels for the (HgX) clusters demonstrate the importance of dispersion interactions in the bonding. Successes are reported for other density functional methods that directly address dispersion.