Do we Really Know How to Define Normal Vibrations in Non-Rigid Molecular Systems?
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The paper analyses ambiguities which exist both in a choice of slowly deforming geometries serving as reference points for vibrations in floppy molecules and in a definition of the large-amplitude internal variables near the selected reference configurations. Since the cited ambiguities change both the frequencies and the shape of normal vibrations, the vibrational analysis becomes an ill-defined problem even in the harmonic approximation. The author formulates some general criteria for selecting a“preferable”coordinate system, which would make it possible to give an intrinsic meaning to the term“harmonic vibrations of a floppy molecule”.
KeywordsInternal Rotation Internal Variable Normal Vibration Harmonic Vibration Reference Path
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