The existence of an electronic current implies the creation of a self-induced magnetic
field, which may in turn back-react on the system. Of course, a fully consistent treatment
of this kind of effect should be done in QED theory, but we will attempt a first
approximation to the problem by considering the lowest-order relativistic terms
plugged into the normal Hamiltonian equations (spin-other-orbit coupling terms, etc.).
For the moment being, none of this is done, but a first step is taken by calculating
the induced magnetic field of a system that has a current, by considering the magnetostatic
approximation and Biot-Savart law:
$ \nabla^2 \vec{A} + 4\pi\alpha \vec{J} = 0$
$ \vec{B} = \vec{\nabla} \times \vec{A}$
If CalculateSelfInducedMagneticField is set to yes, this B field is
calculated at the end of a gs calculation (nothing is done – yet – in the tdcase)
and printed out, if the Output variable contains the potential keyword (the prefix
of the output files is Bind).