The normalized elimination of the small component (NESC) method is a first principles 2-component
relativistic approach that leads to the Dirac-exact description of one-electron systems. It is a powerful method to routinely
investigate chemical and physical properties of molecules containing relativistic atoms. The vibrational modes of mercury
halides are investigated to derive via the corresponding local HgX (X = H, F, Cl, Br, I, At) stretching modes an appropriate
measure for the HgX bond strength. It is shown that HgF bonding in HgF4 is stronger than that in HgF2, which is a result
of enhanced charge transfer from Hg to the four F atoms and the formation of electron deficient 2e-4c bonds with strong
ionic character. A generally applicable bonding model for HgX molecules is outlined.
Keywords: Dirac-exact relativistic methods, local vibrational modes, mercury bonding, Normalized Elimination of the Small
Component (NESC), spin-orbit coupling, .
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