ABINITIO CALCULATIONS OF VIBRATIONAL-SPECTRA OF NONRIGID MOLECULES

The standard quantum chemical treatment (the calculation of gradients, Hessians, and perhaps higher derivatives followed by the harmonic and perturbational treatments of normal modes) of vibrational spectra cannot be applied to nonrigid molecules. We present two examples of nonrigid molecules, propargylene (C3H2) and cyclobutadiene (C4H4), with strongly nonharmonic potential surfaces to which a special treatment must be applied. The two examples are used as a basis for our discussion of the procedure, common features, and computational aspects of the treatments of nonrigid molecules by ab initio calculations. © 1990 American Chemical Society.