 The electrostatic potential is expressed as

where "Z" is the atomic number, "e" is the elementary unit charge, "r" is the distance to the nucleus of the embedded ion and "q" is a screening parameter that determines the range of the potential.The Fourier transform, formula_15, of the lattice potential, formula_16, is expressed as

When the values of the off-diagonal elements formula_15 between the reciprocal lattice vectors in the Hamiltonian almost go to zero.As a result, the magnitude of the band gap formula_19 collapses and the empty lattice approximation is obtained.