  and   stand for the eigenvalues of matrices K and C , respectively. Eq. 27 exposes the difference between the two methods for analysis of a vibrating molecule: the normal modes and the atomic modes. Unlike the normal modes, the eigenvalues  describe the electronic energy exclusively. The number of the atomic modes (n-1) is less than the normal modes (3n-6), indicating that the atomic modes are physically realized by groups of normal modes.