INVESTIGATION OF THE PI ELECTRON SYSTEM OF URACIL WITH DIFFERENT SEMIEMPIRICAL METHODS
UPPSALA UNIV (SWEDEN)
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The pi-electron system of uracil was treated with the aid of the simple Huckel approximation, of the Huckel-CI method, of the semiempirical SCFLCAO-MO method and with the SCF-LCAO-MO-CI method. The excitation energies and oscillator strength values of the first four excited states were calculated. The calculations were performed with two different approximations for the form of the inter-pi-electronic repulsion integrals gamma. The first excitation energy obtained with the aid of the Huckel-CI method gives good agreement with the experimental value in the case of one series of gamma integrals. The SCF calculation in general has given worse results than the Huckel method taking into account the interaction of the pi electrons and a limited CI 4 configurations for the excited states. Since the interconfigurational matrix elements are very small, the CI has not a large influence on the results. The conclusion was drawn that neither of the mentioned semiempirical methods is suitable in its present form for the calculation of the spectra of heterocyclic molecules containing more than one heteroatom. The possibilities for the improvement of the methods are briefly mentioned.
- Atomic and Molecular Physics and Spectroscopy