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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. November 2002, Volume 1, Number 11, 572-582

An MO Theoretical Study of Organic Dyes. II. Comparisons of the Electronic Spectra Calculated by PPP and ab Initio Methods with Various Levels of Theory
Kichisuke Nishimoto
Internet Electron. J. Mol. Des. 2002, 1, 572-582

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Abstract:
The electronic spectra of some typical aromatic hydrocarbons and organic colorants have been calculated by PPP-CI and ab initio methods with various levels of theory. A comparison of calculated results showed that simple PPP-CI calculation with suitable two-center electron repulsion integral sets is successful for the calculation of the π-electronic spectra of organic colorant with high accuracy and also for understanding the nature of their electronic spectra. One can compute molecules having the coronene size within a few seconds using a PC. Thus, this approach is useful for the combinatorial chemistry and for the design of organic colorants. Furthermore, the nature of the π-electronic spectra and the characteristic feature of the lowest excited states, such as ionic, covalent, diradical, etc., are clearly understood by PPP-CI calculations. On the other hand, in some cases, ab initio calculations provide erroneous results and the calculated excited state functions are very complicated. For example, the HOMO-LUMO transition is dispersed over among many excited states. The reason why a modified two-center electron repulsion integral set works satisfactorily is qualitatively discussed.

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