التفاصيل البيبلوغرافية
| العنوان: |
The benzene ground state potential surface. V. Criteria for theoretical modeling of the B2u harmonic force field. |
| المؤلفون: |
Ozkabak, Ali G., Goodman, Lionel, Wiberg, Kenneth B. |
| المصدر: |
Journal of Chemical Physics; 4/1/1990, Vol. 92 Issue 7, p4115, 10p |
| مصطلحات موضوعية: |
NUCLEAR cross sections, BENZENE, PERTURBATION theory |
| مستخلص: |
We demonstrate that fundamental frequencies provide a poor criterion of the benzene B2u force field accuracy and that two-photon cross sections of the b2u fundamental bands in the 1B2u↔1A1g electronic transition, which can be directly related to skeletal displacement magnitudes in the two b2u modes, provide an insightful physical criterion of harmonic force field quality. Another valid criterion for force field quality is isotopic frequency shifts combined with the fundamental frequencies. The frequency-generated force field of part II accurately predicts the measured cross sections and isotopic frequency shifts, indicating that the B2u force constants are known to ±0.01 mdyn/Å. These constants are used as benchmark quantities for calibrating theoretically modeled force fields.A systematic series of ab initio B2u harmonic force fields for ground state benzene using theoretical geometries are generated at Hartree–Fock and correlated second, third, and fourth order (with single, double, triple, and quadruple excitation) Mo\ller–Plesset perturbation theory (MP2, MP3, MP4SDTQ) and configuration interaction theory with all single and double excitation (CISD) levels using basis sets from minimal double zeta to triple zeta plus diffuse and polarized functions. These theoretical models of the B2u force field all provide poor predictions for the three criteria: fundamental frequency accuracy 2%–3%; isotopic frequency shift accuracy 10%–300%; two-photon cross section accuracy 300%–1200% with the sense of isotopic effects on two-photon cross sections in some cases incorrectly predicted. The MP2 calculations, even using the largest basis set, are incapable of meeting any of the criteria, hence higher order approaches to the correlation problem are required. The inadequacies in frequencies, isotopic shifts, and mode forms arise because both the diagonal and off-diagonal force constants are not predicted by ab initio... [ABSTRACT FROM AUTHOR] |
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| قاعدة البيانات: |
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