159 elements only between states which differ in one of the single-electron wave functions, in short, HeR induces only one-electron transitions. The matrix elements 1mn and Pmn reduce to matrix elements between the single-electron wave functions. We are interested primarily in crystalline solids for which the band model is a good approximation. The Bloch single-electron wave function in this model has the form: N'I ili-';; U. r.;;) ( (1.14) ""nk r,...