QCISD level

Optimize the structure of acetyl radical using the 6-31G(d) basis set at the HF, MP2, B3LYP and QCISD levels of theory. We chose to perform an Opt Freq calculation at the Flartree-Fock level in order to produce initial force constants for the later optimizations (retrieved from the checkpoint file via OptsReadFC). Compare the predicted spin polarizations (listed as part of the population analysis output) for the carbon and oxygen atoms for the various methods to one another and to the experimental values of 0.7 for the C2 carbon atom and 0.2 for the oxygen atom. Note that for the MP2 and QCISD calculations you will need to include the keyword Density=Current in the job s route section, which specifies that the population analysis be performed using the electron density computed by the current theoretical method (the default is to use the Hartree-Fock density). 

Two transition structures were found at the MP2/6-31G level. The difference in the CO distances is given for a Markovnikov-type structure. While Markovnikov-type transition structures (albeit much more symmetrical) were found at the B3LYP and QCISD levels, this MP2 TS is 0.2 kcalmoL higher than an anti-Markovnikov TS (for which the distance difference is -0.363 A). 

B3LYP level, 4, 34-5, 38, 40, 41, 43 CCSD level, 3, 34-5, 43 dioxirane oxygen atom transfer, 34-5 QCISD level, 3, 34-5, 43 10-Deoxoartemisinin, synthesis, 289-90 10-Deoxoartemisitene, synthesis, 289-90 Deoxyartemisinin, not antimalarial, 1280-1, 1283... 

In a similar spirit, Poater et al. have reoptimized the B3LYP parameters in order to minimize differences between computed electron densities from this modified DPT level and calculated at the QCISD level for a series of 16 small molecules (Poater, Duran, and Sola 2001). They observed, as already emphasized above, that different molecules require different amounts of exact HF exchange for optimal agreement between the two methods. 

Figure 1. HF-GIAO 19F absolute chemical shieldings for C6H5F (in ppm) using (/) a 6-31G basis on all atoms and (//) a locally-dense basis (6-31G on C and H, and 6-311++G(2d,2p) on F) and geometries optimized at the HF, CASSCF(6,6), MP2, MP4(SDQ) and QCISD levels of theory (all within the 6-31G basis).

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 ... 

The calculated structures (at the QCISD level with triple- basis sets which include diffuse and polarization functions for Si, Ge and Sn, and ECPs for Pb)168 of D3h MII5 group 14 anions, which are all minima on the PES, are given in Figure 40. The M—H... 

In contrast, trans-2 + has a C2h symmetry and a 2Bg ground state, therefore in principle allowing a symmetry preserving pathway to correlate it with the ground state of 1 +. The corresponding C2-symmetric transition structure has, at the QCISD(T)//QCISD level of theory and using a 6-31G basis set, an activation energy of 23.4 kcal/mol. However, a frequency analysis of this transition structure... 

The experimental vibrational harmonic frequencies cue for Au Cl (and Au Cl equal to 382.8 cm (and 373.9 cm , respectively) and anharmonicity constants coupled cluster procedure theory QCISD(T) which gives cue values of 369.5 cm (and 360.9 cm ) and weXe values of 1.32 cm (and 1.26 cm ), respectively, for Au Cl (and Au Cl) isotopomers. The estimated " dissociation energy of 3.0 0.7 eV and the value of 2.85 estimated at the QCISD level indicate that the AuCl dissociation energy should be below 3.5 eV and that the experimentally obtained value" of 3.5 0.1 eV is probably overestimated by about 0.5 eV. The vibrational-state dependencies of the molecular properties for Au Cl have been established (equations 77-79) ... 

Rayon and Sordo located a set of BCPs in extrema of the potential energy surface of the C2H2...SO2 complex explored at MP2 and QCISD levels with Pople and Dunning basis sets. Three classical properties evaluated at the S... 0, S... Jt and O... H BCPs are reminiscent of van der Waals molecules. 

Insertion of SiH2 into F2 has been examined with a 6-31G basis set and full geometry optimization at the MP2, CCD, CISD and QCISD levels ". Two transition states were found, differing by whether one of the F atoms approaches syn or anti to the Si lone pair. For anti approach there was no baniCT found, while for syn approach a complex was found, whose energy is 2.5 kcalmol below the reactants while a barrier of 0.5 kcalmol separates the complex from the products. 

To illustrate how well DFT or ab initio methods predict the dipole moments. Table 1 illustrates the comparison between theory and experiment for eight small molecules. The error statistics are summarized in Table 2. In general, the quality of the basis set plays an important role in the prediction of dipole moments. We see that the 6-3IG basis set provides poor predictions, even when applied with a QCISD level of theory. The performances of the double-zeta basis set plus polarization functions (6-3IG, DZVPD (double-zeta valence orbitals plus polarization and diffuse functions on heavy atoms), and cc-pVDZ (correlation-consistent polarized valence double-zeta)) are poorer than those from the polarized triple-zeta basis sets. The only exception is B-P/DZVPD (B-P = Becke-Perdew), from which we obtained an average absolute deviation of 0.040 debye, lower than that (0.053 debye) from B-P/TZVPD (triple-zeta valence orbitals plus polarization and diffuse functions on heavy atoms). It can be seen that the inclusion of correlation effects through either ab initio or DFT approaches significantly improves the agreement. 

Table 1 Effects of Basis Set Size and Oxygen s-Shell Deconttaction on the Isotropic hfcc (Gauss) of OH. All Calculations are Performed at the QCISD Level, Using the MP2/6-31G(d) Optimized Geometry (R = 0.979 A). From Ref. 12 and this Work...

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