Chapter 15. External Electromagnetic Fields and Molecular Properties

Summary

The relativistic calculation of observables and molecular properties follows the lines elaborated first in nonrelativistic quantum chemistry, in which the small electromagnetic perturbation is considered in a Taylor expansion of the electronic energy. Molecular properties are then defined as the derivatives in this expansion. However, the relativistic minimal coupling principle allows for a rigorous inclusion of electromagnetic fields. Thus, four-component theory of response properties derived from this perturbation is naturally a consistent framework. Such a theory, however, requires the perturbed wave function, which is usually expanded in terms of the set of eigenfunctions of the unperturbed Hamiltonian so that the role of the negative-energy states has to be clarified. In particular, they become decisive for the explanation of the diamagnetic contribution of magnetic-response parameters. Again, two-component schemes are free of negative-energy states, but this benefit needs to be analyzed carefully. Also, they suffer from the so-called picture-change error.