Density functional theory QCISD level

Ab initio (HF, MP2, and MP4) (99JCS(P2)801) and SCF + MBTP(2) methods (90JPC7406) also predict the predominance of the thiol tautomer in the gas phase however, the relative stability of the thiol tautomer is overestimated by 1 kcal/mol and 6.5 kcal/mol, respectively. Similarly, calculations using large (TZV2P) basis sets and electron correlation at the QCISD(T) level overestimate the thiol stability by 3.6 kcal/mol (93JCS(P2)861). In contrast, the density functional theory (B3LYP)... 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Pending ongoing developments of improved functionals, an effective multi-scale scheme (sketched in Figure 2.3) can be profitably used, where the NO moiety is treated at the Quadratic Configuration Interaction Single and Double (QCISD) level of theory and the remaining parts of the system are treated by means of hybrid density functionals ... 

In another study, G3MP2, G2, G3, G2MP2, G3B3, G3MP2B3, QCISD(T), BS-4, CBS-Q, CBS-QB3, and CBS-APNO, produced AGg values for nitrous acid within 0-1.6 kcal/mol [77] of the experimental value of 333.7 kcal/mol [78]. They also found that the less expensive density functional B3LYP produced values within 2.72 kcal/mol of experiment The commonly used Hartree-Fock level of theory, which does not include correlation energy, produced inaccurate results with a large 4.66 kcal/mol discrepancy [77]. 

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