  X-ray structure factors from four-component molecular wave functions have
been calculated for the model systems $M$(C$_2$H$_2$) ($M$= Ni, Pd, Pt).
Relativistic effects on the structure factors are investigated by the
comparison to results obtained from a non-relativistic reference and in order
to systematically analyse the effect of different quasi-relativistic
approximations, we also included the DKH2 and the ZORA Hamiltonian in our
study. We show, that the overall effects of relativity on the structure factors
on average amount to 0.47, 0.80 and 1.27% for the three model systems under
investigation, but that for individual reflections or reflection series the
effects can be orders of magnitude larger. Employing the DKH2 or ZORA
Hamiltonian takes these effects into account to a large extend, reducing the
according differences by one order of magnitude. In order to determine the
experimental significance of the results, the magnitude of the relativistic
effects on the structure factors is compared to the according changes due to
charge transfer and chemical bonding.
