Ab Initio Calculations of Electronic Absorption Spectra

These methods lead in general to very good agreement between calculated and observed quantities. Rydberg states are also described well. They are characterized by high quantum number orbitals, which are much more extended than valence orbitals. Molecular Rydberg orbitals resemble atomic orbitals since an electron in such an orbital is so far from the nuclei of the 

State CP MRDCP Effective Valence ShelP MCSCF CP CASSCF PT2i Exp. 

For jr—excitation energies the agreement between the calculated and experimentally observed data is more difficult to achieve. This is probably related to the intricacies of the correlation of a with Ji electrons in states with a highly zwitterionic character in the n part of the wave function. (Cf. Malmqvist and Roos, 1992.) Some of the difficulty is due to the limited availability of definitive and accurate experimental data. As an example, some data for butadiene are collected in Table 1.6. 

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In the past decade, vibronic coupling models have been used extensively and successfully to explain the short-time excited-state dynamics of small to medium-sized molecules [200-202]. In many cases, these models were used in conjunction with the MCTDH method [203-207] and the comparison to experimental data (typically electronic absorption spectra) validated both the MCTDH method and the model potentials, which were obtained by fitting high-level quantum chemistry calculations. In certain cases the ab initio-determined parameters were modified to agree with experimental results (e.g., excitation energies). The MCTDH method assumes the existence of factorizable parameterized PESs and is thus very different from AIMS. However, it does scale more favorably with system size than other numerically exact quantum... 

The structures of the pairs have been determined by ab initio calculations. Surprisingly, while the absorption spectrum of the solvated electron presents a single band located around 2250 nm, the absorption spectra of the pairs are blue-shifted and composed of two bands (Fig. 7)7 Those spectra were interpreted as a perturbation of the solvated electron spectrum with the use of an asymptotic model. This model describes the solvated electron as a single electron trapped in a THF solvent cavity and takes into account the effects of electrostatic interaction and polarization due to the solutes that are modeled by their charge distribution. It was shown that the p-like excited states of the solvated electron can be split in the presence of molecules presenting a dipole. So, the model accounts for the results obtained with dissociated alkali and non-dissociated alkaline earth salts in THF since ionic solutes yield absorption spectra with only one absorption band, and dipolar neutral solutes yield absorption spectra with two bands (Fig. 8). ... 

Helium photoelectron spectra of the four thiadiazoles have also been determined and have been interpreted by a comparison with those of thiophene and azole, and by ab initio MO calculations. Information is provided concerning the electronic charge distribution, and the electronic absorption spectra associated with these structures. The results suggest that... 

Lanthanide compounds have attracted attention due to their potential applications as various materials. Their intra-4f electronic transition, the so-called f-f transition, is one of the most discriminative properties and used extensively for many optics, such as lasers, fibers, optical displays, and biosensors [1-3]. The probability of photo absorption is represented by oscillator strength, a dimensionless quantity. The oscillator strengths of f-f transitions are small, typically in the order of 10 , reflecting Laporte forbidden transitions, and their absorption and emission spectra have sharp peaks in visible, near infrared, and near ultraviolet regions. Despite the usefulness of these systems, there are only a few relevant ab initio studies of the f-f transition intensities that explicitly included ligand electrons [4—7] because these calculations are difficult due to the importance of both relativistic and electron correlation effects. 

The antisymmetric stretching vibration in the electronic ground state X Bi, V3 = 3359.932(2) cm", was obtained from an IR absorption spectrum [1]. Additional vibrational frequencies of NH2 in the three lowest electronic states were determined from photoelectron spectra (PES) of NH2(X Bi) [6]. Vibrational frequencies of NH2 and Isotopomers (in cm" ) from experiments (see data collections [7 to 9]) and ab initio calculations are listed in the following table ... 

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