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Difficulties in application of the orthogonalized plane wave method of determining energy levels in solids may arise under two conditions: If (1) the potential around an atom in the crystal is appreciably different from that of a free atom; or if (2) the wave function for a valence electron state is orthogonal by reasons of symmetry to all core states. Ways of treating these problems are discussed, and applied to two eigenvalues in iron.
J. Callaway (1955) studied this question.