ABSTRACT
Molecules and crystals are made of two kinds of particles with very different masses, light electrons and heavy nuclei, both moving in the fields generated by Coulomb forces. Different masses entail different scales for the space variation of the wave functions and this is the basis of an approximation, the Born-Oppenheimer approximation, widely used in the theory of complex atomic systems. A recent illustration of the Born-Oppenheimer approximation in QED is found in Weinberg's book on Quantum Mechanics [101].
