Relativistic all-electron molecular dynamics simulations.

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