QCISD, density functional theory

QCISD, density functional theory, 3, 34-5, 43 Qinghaosu see Artemisinin (qinghaosu) Quahty, deterioration evaluation, 656, 664 Quantum chemical calculations,... 

Methods based on Density Functional Theory also include some electron correlation effects (we ll consider them a bit later in this chapter). Of the traditional post-SCF methods, we ll be primarily using MP2, MP4, QCISD and QCISDfO in this work. 

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

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)... 

B3LYP = hybrid Hartree-Fock and density functional theory using Becke s three-parameter formula and the Lee-Yang-Parr correlation functional QCISD(T) = quadratic configuration interaction singles, doubles, and triples. 

Molecular orbital theories (ab initio methods (23)) were chosen and validated in this study to characterize the interaction energies between methane and water, while electron density functional theories (24) were tested, but found to be inadequate (see below). Four different ab initio methods were used in the validation MP2 (25,26), MP4(SDTQ) (27), QCISD(T) (28) and CCSD(T) (29). Three different DFT methods, BLYP (30Jl), B3LYP (32) and BPW91 (33), were used and the results compared with the ab initio methods. In addition, for each of the above methods, the effect of the size of different basis sets was investigated specifically, 6-31++G(2d,2p), cc-pVDZ, cc-pVTZ and cc-pVQZ were used. 6-31+-i-G(2d,2p) was chosen, because it was reported to yield reasonable results compared with that at near the basis set limit on this system (77). The others were chosen in order to observe the effect of systematically increasing the size of the basis set. Gaussian 94 (22) was used for all the above calculations. 

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 practice, using currently available exchange and correlation potentials, this path leads to results [113] worse than those obtained with the Hartree-Fock method. This is illustrated for momentum moments in Table 19.2 which shows median absolute percent errors of (p ) for 78 molecules relative to those computed by an approximate singles and doubles coupled-cluster method often called QCISD [114,115]. The molecules are mostly polyatomic, and contain H, C, N, O, and F atoms. The correlation-consistent cc-pVTZ basis set [110] was used for these computations. Table 19.2 shows the median errors for the Hartree-Fock method, for second-order Mpller-Plesset permrbation theory (MP2), and for DFT calculations done with the B3LYP hybrid density functional... 

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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