Semiempirical approximations, optical

Now we want to discuss IR optical spectra of the C60H36 synthesized at high-pressure. Results of this study were published in Bazhenov et al. (2008). There are a lot of publications devoted to theoretical and experimental study of C60H36. We should pay attention on the existing discrepancies in the results of theoretical calculations of the dipole-active spectra C60H36, compare, for example, papers Bini et al. (1998) and Bulusheva et al. (2001). There were used different theoretical models. Semiempirical method of the MNDO type (Dewar and Thiel 1977) was used in (Bini et al. 1998). Ab initio Hartree-Fock self-consistent field approximation was used in (Bulusheva et al. 2001). 

One such link between semiempirical theory and experiment that appeared about that time was the development of calculational methods for optical rotatory dispersion. Moffitt s theoretical work with Kronig—Kramers transforms coupled with Djerassi s experimental data on steroids gave rise to rules for the prediction of the sign of optical rotation. Computer calculations with semiempirical methods played a role. i Wavefunctions of at least an approximate sort were needed for the dipole and dipole velocity matrix elements of the theory. 

Fig. 1 Size dependence of optical gaps of silicon nanocrystals calculated using quantum Monte Carlo (QMC), time-dependent local-density approximation (TD-LDA), Hartree-Fock configuration interactions (HF-CI), and semiempirical tight binding (TB). The inset shows schematically the bandgap enlargement due to reducing die nanocrystal size...

All of the semiempirical treatments described thus far fall under the rubric of the molecular orbital (MO) approximation. To charaaerize optical speara, one must, in general, also include the interaaion between excited elearon configurations obtained by promotion of one or more elearons from occupied to virtual orbitals. Configuration interaaion (Cl) was first introduced into the PPP method and, then, generalized to all valence electrons in 1968 by Del Bene and Jaffe. The latter method, called CNDO/S, is more similar to PPP in the calculation of elearon repulsion integrals and more flexible than either PPP or CNDO/2 in the treatment of resonance terms. The extension to INDO/S has also been made, and all three Cl approaches have achieved some important successes. 

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