For considering the electron-phonon coupling in the problem of impurity absorption band, a set of normal modes of lattice vibration were formed by the linear combination of plane (running) waves in order to present fully the site symmetry of the impurity. The symmetric (standing) waves of a definite type are characterized by an irreducible representation of the site group. The frequency distribution function of the phonons of a certain symmetry type is that of the lattice vibration divided by a constant. The phonons involved in the transition of an absorption band are restricted to certain symmetry types but no selection on the frequency may be found. The configuration coordinate approximation of Williams et al. was critically examined at this stage. The dependence of the coupling strength on the frequency of phonons was qualitatively discussed and the possible importance of the critical points of the frequency distribution was remarked. The mathematical analysis of symmetric standing waves was given in details. The relation between this set of normal modes of lattice vibration and those of an isolated cluster containing a central ion and the ligands was demonstrated.