The Scaled Quantum-Mechanical Force Field - A promising approach to the molecular vibrational dynamics

We present the physical backgrounds of the Scaled Quantum-Mechanical Force Field (SQMF) approach to the vibrational dynamics of molecules. SQMF is a semi-empirical method, which is based on a preliminary quantum-mechanical calculation of the equilibrium geometry of the molecule together with its normal modes and eigenfrequencies. Subsequently a specific scaling transformation is applied to the as determined molecular force-constants in order to reproduce a set of empirically observed vibrational frequencies. SQMF being based on the realistic electronic structure of the molecule removes many of the ambiguities specific to the purely empirical valence force field approaches. We discuss also the problem of attribution of the experimentally observed frequencies to the calculated normal modes. In this context we propose a new technique, the so-called Maximum Spectral Overlap (MSO), which is especially helpful in the analysis of the Raman and IR spectra of multicomponent samples or molecules with low symmetry. The SQMF technique is illustrated with two sample systems: the 133 polyaniline oligomer and its ionized form B3(+.).