Combined Plane-Wave Tight-Binding Method for Energy-Band Calculations with Application to Sodium Iodide and Lithium Iodide

Abstract

A method of calculating energy bands, which we name the mixed-basis method, is developed. This method uses a wave-function expansion which includes free-atom or ion wave functions and plane waves, and it may incorporate relativistic effects. It is, in general, not equivalent to the orthogonalized-plane-wave (OPW) method, although the cases for which the two methods are equivalent are developed. A sample calculation is made for LiI and NaI, for which x-ray, valence, and conduction states are calculated. The agreement between theory and x-ray and optical-absorption data is excellent. The importance of relativistic effects is explored, and comparisons are made with conventional OPW results, which include relativity by first-order perturbation theory.