  Thermal vibrations alter the external potential. Allen (Phys. Rev. B 18
(1978) 5217) proved that at finite temperatures the pseudopotential form
factors are corrected by a Debye-Waller Factor (DWF). We generalize this result
to the crystal potential. (The generalization to the all-electron case of the
nuclear potential fails due to the breakdown of the rigid-atom approximation.)
This finite temperature formalism only gives thermal-averaged properties and no
dynamical information can be obtained. Hence, it is labeled the Quasi Ab Initio
formalism. Analogous to the use of experimental lattice parameters in ab initio
studies, experimental DWF can also be used. The justification is identical; the
experimental parameters can be validated by separate ab initio studies. Our
work transforms, forty years later, Kasowski's empirical study (Phys. Rev. B. 8
(1973) 1378) into the first ab initio finite temperature band structure
calculation. This formalism opens the way to obtain ab initio finite
temperature thermal-averaged properties from a single calculation.
