How to Describe the Vibrations of a Molecule

The main purpose of a dynamical analysis of a molecule is to assign the vibrational transitions observed in infrared and Raman in terms of molecular motions. As mentioned above, chemical group frequency correlations have been the justification of most of the vibrational assignment. The description of the normal modes has posed some problems throughout the years. 

It would seem obvious that the only way to describe the atomic motion would be that of plotting the cartesian displacements of each atom during normal mode Qj with frequency vj. At present, with the availability of computer techniques and fully automated computing programs, the cartesian atomic displacements are a normal part of the output, while some programs even provide animated descriptions of the molecular vibrations which can be viewed in all directions by appropriate use of 

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