MRCI studies

Parusel ABJ, Kohler G (2001) Influence of the alkyl chain length on the excited-state properties of 4-dialkyl-benzonitriles. A theoretical DFT/MRCI study. Int J Quantum Chem 84(2) 149-156... 

Marguet, S., Mialocq, J. C., Millie, P., Berthier, G. and Momicchioli, F. Intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye in polar solvents. A CS-INDO MRCI study,... 

One interesting development that took place during the MRCI studies was the announcement by Grev and co-workers that their CISDTQ results were incorrect [90]. Using correct CISDTQ values to revise their adjustments to the CISD+Q results they obtained an estimate of 4+4 kcal/mol for the separation. This was now consistent with the assertion of Martin and co-workers that it was not possible to decide whether the minimum energy structure was cyclic or linear. 

However, while the CASSCF method can handle the near-degeneracy problem in a balanced and effective way, it does not include the effects of dynamic (external) correlation. For small molecules it is possible to treat this problem using multireference (MR) Cl or similar techniques. Two recent review articles in the electronic spectroscopy of diatomic [3] and triatomic [4] molecules provide good illustrations of the present state of the art. The MRCI method is in principle capable of very high accuracy. It is also very general and can treat any type of electronically excited state. However, the computational effort increases steeply with the number of correlated electrons. It is therefore not a method that can be used for larger molecules. An illustration is given by a recent MRCI study... 

The theoretical studies on imidazole are scarce. A recent MRCI study by Machado and Davidson [137] considered valence and Rydberg states together for the first time. On the other hand, the available experimental spectra of imidazole are limited to methanol, ethanol, and aqueous solutions and it is difficult to establish the effects of the solvent on the different transitions. The structure of the computed gas-phase spectrum differ from the spectra in a solvated medium. An unambiguous assignment of the main valence bands in the spectrum of imidazole thus requires consideration of such effects. The reaction field method, explained above, was employed with the CASSCF/CASPT2 method to analyze the influence of the solvent on the valence states of... 

The electronically excited singlet states of uracil, and how they can lead to efficient radiationless decay to the ground state, were initially investigated using MRCI methods by Matsika [147] and later with other methods that agree with the MRCI results in the more general features [92, 126, 128, 148-150], The discussion in this section describes MRCI results for free uracil in detail, along with the studies that have been... 

The CASSCF/CASPT2 calculations were performed with an active space including the five nd, the (n + l)s, the three (n+ l)p orbitals, and a second set of nd orbitals to account for the double shell effect. The importance of including a second 3d shell in the active space was detected in an early study of the electronic spectrum of the nickel atom [2]. This had already been suggested from MRCI results [1]. The results obtained by RT at about the same time indicated that such effects are effectively accounted for when a method is used that includes cluster corrections to all orders, like the QCI method used by them [3]. This result will hold true also for the less approximate coupled cluster method CCSD(T). 

Comparison with experimental values are meaningful only for boron and aluminum. The MRCI values for AI (0.45 eV) and the MCDF results for B (0.26 eV) and A1 (0.43 eV) are in good agreement with the Hotop and Lineberger values (0.28 and 0.44 eV, respectively). The MRCI and MCDF EAs for the other atoms agree with each other (0.29 and 0.30 eV for Ga, 0.38 and 0.39 eV for In, 0.27 and 0.29 eV for Tl). The RCC EA of TI is much higher at 0.40(5) eV. A major difference between the RCC and the other two methods lies in the number of electrons correlated. While [52] and [53] correlate valence electrons only, three for the neutral atom and four for the anion, we correlated 35 electrons in Tl and 36 in TE. A RCC study of all five elements was undertaken [54], with the aim of determining all five EAs and, in particular, the effect of inner-shell correlation and virtual space used on the calculated values. 

The semiempirical method used in this study is the OM2/MRCI method developed by Thiel and coworkers [39, 40], In the OM2 method, proper orthogonality of the orbitals has been introduced, which leads to a better description of energy... 

It would be elegant, but disingenuous, to present this study in a logical order the true sequence of events was much less logical and it is more honest to present that here. The original motivation was to examine the trimer and tetramer of beryllium (and magnesium) with the CCSD method, and to compare the results with those from MRCI. The work was started in 1988, well before we had the capability of including any effects of connected triple excitations in the coupled-duster treatment. 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

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