Adequacy of Local‐Exchange Excitation Hamiltonias in Insulators

The local‐density (Xα) calculation of insulator band gaps is re‐examined. Both new and extant data show the calculated gaps to be systematically small by substantial percentages (35 to 50%) while cohesive energies, valence band widths, and PV curves are in satisfactory agreement with experiment. The Slater transition state cannot be invoked directly to resolve the dilemma and its periodic generalization is unsuccessful. The simple periodic excitation calculation of Δ E SCF can be interpreted usefully in terms of the first exciton absorption, a fact we illustrate in f.c.c. Ar. It is argued that the numerical results indicate strongly that Xα calculations of metal‐insulator transition pressures yield lower bounds to that pressure.