MAGNETIC SCREENING OF NUCLEI BY ELECTRONS AS AN EFFECT OF GEOMETRIC VECTOR POTENTIAL

It is known that the usual Born-Oppenheimer procedure,when applied to a molecular system in an external magnetic field, leads to an effectiv e nuclear Hamiltonian in which the magnetic field acts on bare nuclei, so that, for instance, a neutral atom or molecule would be deflected in such a field as if the nuclei were the only charged particles. Elab orating on previous work, by others as well as ourselves, we discuss h ow this situation can be remedied by including the effect of the geome tric phase and geometric vector potential. In particular, it is shown that inclusion of the geometric vector potential leads to results in a ccord with intuition and semiclassical calculations in simple cases, a nd that for all cases it provides a consistent Born-Oppenheimer-type f ormulation, free of obvious defects, for a molecular systemin an exter nal magnetic field.