not
possible to perform elaborate electronic structure calculations,
but they succeeded in describing the one-electron energy
bands derived from empirical spectral terms of the separate
ions. In forming a crystal, the energies of the ionic states
change due to the Madelung potential and bands broaden due
to the interaction between wave functions centered on neighboring
ions. Slater and Shockley argued that the lowest unoccupied
levels in NaCl were formed by hybridization of
states which originally belonged to the continuum of excited
levels of a Cl2 ion with Na 3s wave functions. It is remarkable
that these authors were intuitively aware of the importance
of Cl states for the conduction band