ABSTRACT
This chapter provides some length the special problem of exchange, symmetry, and the real nature of the solid state as viewed from a quantum point of view. The type of correction, anharmonicity, is very important in trying to achieve a quantitative understanding of solid helium. In general, in the realistic case, the ladder will stop at a finite energy and be replaced by a continuum, and in fact, in the helium case, there are only one or two bound excited states at most. The Einstein-Hartree theory that is most completely worked out for the heliums is not this second-quantized, corrected Hartree theory, but a rather similar exercise called the "method of correlated basis functions". Hartree-Fock approximation since the potential hole does not move with the atom. Apparently the hole becomes mobile only when three-boson excitations are taken into account.
