G3 Theory Smic Electronic And Multi-reference State Exact Algorithm,

The theoretical procedures of the Gaussian-3 theory and several of its simplified or modified models as well as the various modifications on the G2 theory were reviewed in detail. Reliability comparisons on most of the models were included and ten directions of the problems on further improvement of the accuracy of the accurate model chemistry were proposed. Several modifications to the G2 and G3 theories on core electronic correlation, geometry optimization and the reliability in the non-equilibrium molecular energy calculations from our recent work were also reviewed. Based on the review, we carried out the study on the problems of core electronic correlation and of single-reference states approximation. Core electronic correlation is necessary for accurate calculations. We obtained the result that the level of theoretical method has little effect on the core electronic correlation in the reaction energy calculations for molecules at equilibrium structure. This predicts that the lowest level of post-Hartree Fock method of MP2 could be used. However, increasing of the size of basis sets would increase the accuracy of reaction energy calculations. Multi-reference states technique is the only way of taking account the non-dynamic correlations and therefore has been widely used in the study of reaction kinetics and of excited problems in the recent years. Nevertheless, the accuracy remains unknown in the literature. We have also tried to obtain the accuracy of MR-CJSD in the general reaction energy calculations. In the case of lacking experimental data for molecules at their non-equilibrium structure, we have to use and calculate the data for molecules at their equilibrium structure. It is shown that the accuracy of MR-CISD plus the High Level Correction has the same accuracy as that from the well-known model chemistry G2 or G3 theory. The latter is based on the single-reference state approximation and could be used correctly only on the problems of non-dynamic correlation being negligible. Therefore the present results provided useful information for the research of molecules having larger non-dynamic correlation effects. We have also carried out some researches on the related problems in Multi-reference states calculations, e.g. selection of MR states and assignment of the electron occupations. The perturbation states selection from the MELD is superior to the others, such as CASSCF and CISD. We have determined most of the electronic occupations on molecules of the G2-l Test via the applications of group theory.