Theoretical and Interpretative Paradoxa
of Different Relativistic and Quasirelativistic Approaches
W.H.Eugen Schwarz
Theoretische Chemie, Universität Siegen, D-57068 Siegen, Germany
The standard ab-initio approach in relativistic quantum chemistry rests
on the Dirac equation, using the standard four-component spinor
representation in ordinary (position or momentum) space. Then, r denotes
the position of charge, and p=-id/dr the mass momentum. By unitary
transformations one can pass over to effective Hamiltonians, to
two-component representations, to pseudopotential or to one-component
approximations. Thereby the expressions for expectation values, for the
charge distribution, for the optical transition moment, for spin-orbit
coupling etc., and the expressions for relativistic corrections change.
Accordingly one must be careful when interpreting relativistic changes
of structure, forces and energy of molecules. They may be explained
through changes in the valence shell or in the inner core, using
relativistically corrected or nonrelativistic wavefunctions, using
forces or energy gradients. Combining different paradoxical viewpoints
yield a clearer understanding of the mechanism of relativistic
corrections.