Density functional theory CCSD level

The vibrational spectrum of 13 is nicely reproduced by quantum chemical calculations at the CCSD(T) level of theory," whereas density functional theory (DFT) shows a variable performance for 13 depending on the functional employed." This caution holds especially for the distance between the radical... 

Quantum chemical calculations of the Ne potential-energy hypersurface have shown that the qualitative shape shongly depends on the choice of the theoretical method and basis set. All the geometries represented in Scheme 6 have been shown to be minima on the potential smface, but most of them do not possess minima at all the levels of theories applied. Hexaazabenzene (13), for example, has a minimum for a stmctme at the HF level of theory. However, this geometry is a second-order saddle point with the density functional theory (DFT) and also at the MP2 level of theory. D2 hexaazabenzene has a minimum structure at DFT, but at the CCSD(T)/aug-cc-pVDZ level, the D2 geometry resembles a van der Waals complex of two N3 units, whereas it is a minimum structure at the CCSD(T)/cc-pVTZ level of theory. Similar behavior has been observed for most of the other isomers. 

Over the years, several computational methods have been developed. The variational theory can be used either without using experimental data other than the fundamental constants (i.e., ab initio methods) or by using empirical data to reduce the needed amount of numerical work (i.e., semiempirical data methods). There are various levels of sophistication in both ab initio [HF(IGLO), density functional theory (DFT) GIAO-MP2, GIAO-CCSD(T)] and semiempirical methods. In the ab initio methods, various kinds of basic sets can be employed, while in the semiempirical methods, different choices of empirical parameters and parametric functions exist. Tire reader is referred to reviews of the subject. ... 

The latest MCPs for the main group elements [97] were calibrated against experimental structural data in careful studies to assess their reliability [210]. All the recent basis sets were used (MCP-dzp, MCP-tzp, and MCP-qzp) together with several methods of varying sophistication post-Hartree-Fock (MP2 [211-216] and CCSD [217]) and density functional theory (PB86 [218, 219], PBE [220], and PBEO [221]). It was found that the level of theory that is capable of providing reliable structural data is DFT/MCP-tzp. The MCP-dzp parameterization is most suited for qualitative studies. For the post-Hartree-Fock methods (MP2, CCSD) the basis set of MCP-qzp quality is required for quantitative studies. The results, summarized in Figs. 8.9 and 8.10 show that the MCP basis sets behave similarly to their all-electron counterparts [222] (see also Chap. 15 of Ref. [223]). 

Fig. 7. Distance-dependent potential energy curves for the anunonia adduct to the silyl cation, H3N"-SiH3 at different levels of correlated wavefunctions density functional theory (DFT-B3LYP) with a basis set of triple-zeta quality (aug-cc-pVTZ), MP2 as well as MP4 perturbation, and coupled cluster CCSD(T) calculations with double-zeta basis sets (aug-cc-pVDZ).

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