The scalar-relativistic Douglas–Kroll–Hess method is implemented in the Born–Oppenheimermolecular dynamics simulation package CP2K. Using relativistic densities in a nonrelativisticgradient routine is found to be a valid approximation of relativistic gradients. An excellentagreement between optimized structures and geometries obtained from numerical gradients isobserved with an error smaller than 0.02 pm. Hydrogen halide dimers (HX)2, with X=F, Cl, Br, Iserve as small test systems for first-principles molecular dynamics simulations. Relativistic effectsare observed. That is, the amplitude of motion is larger, the frequency of motion is smaller, and thedistances are larger in the relativistic picture. Several localization schemes are evaluated fordifferent interatomic and intermolecular distances. The errors of these localization schemes aresmall for geometries which are similar to the equilibrium structure. They become larger for smallerdistances, introducing a slight bias toward closed packed configurations.
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