Basis B3LYP

Figure 3 Errors in potential energies for CH4 in a 6-3IG basis. B3LYP and UB3LYP error curves have been shifted up by 70 kcal mof. ...

For compounds 125 and 126, the experimental values of the enthalpy of combustion are given, AHf° (gas) are calculated at the basis B3LYP/6-31G, and the detonation velocity is calculated by the method of Kamlet and Jacobs <2006JOC1295>. 

MRCI computations (d-aug-cc-pVDZ AO basis, B3LYP/TZV(d,p) ground-state geometry) are compared with experimental data in Table 5. A graphical comparison with an experimental gas-phase absorption spectrum is shown in Figure 12. 

Figure 20 Comparison of the experimental and computed [TZV(d,p) AO basis, B3LYP optimized geometry] CD spectrafor ( M)-[6]helicene. The theoretical spectra have been shifted by 0.20 (B3LYP) and —0.22 eV (CC2), respectively. The filled circles/triangles indicate the two lowest states with small intensity obtained by CC2/TDDFT-B3LYP. The vertical lines correspond to results from the CC2 method.

In hybrid DET-Gaussian methods, a Gaussian basis set is used to obtain the best approximation to the three classical or one-election parts of the Schroedinger equation for molecules and DET is used to calculate the election correlation. The Gaussian parts of the calculation are carried out at the restiicted Hartiee-Fock level, for example 6-31G or 6-31 lG(3d,2p), and the DFT part of the calculation is by the B3LYP approximation. Numerous other hybrid methods are currently in use. 

We win run this job on methane at the Hartree-Fock level using the 6-31G(d) basis our molecule specification is the result of a geometry optimization using the B3LYP Density Functional Theory method with the same basis set. This combination is cited... 

In order to do so, you will need to perform Hartree-Fock NMR calculations using the 6-311+G(2d,p) basis set. Compute the NMR properties at geometries optimized with the B3LYP method and the 6-31G(d) basis set. This is a recommended model for reliable NMR predictions by Cheeseman and coworkers. Note that NMR calculations typically benefit from an accurate geometry and a large basis set. 

You should be aware that the optimal scaling factors vary by basis set. For example, Bauschlicher and Partridge computed the B3LYP/6-311+G(3df,2p) ZPE/thermal energy correction scaling factor to be 0.989. Additional scaling factors have also been computed by Wong and by Scott and Radom. 

Perform a low-level geometry optimization with a medium-sized basis set, for example, a Hartree-Fock or B3LYP Density Functional Theory calculation with the 6-31G(d) basis set. (For very large systems, a smaller basis set might be necessary.)... 

The optimized bond length in PO will serve to illustrate the effect of larger basis sets. Here are the predicted values for several medium and large basis sets (all optimizations were run at the B3LYP level of theory) ... 

Determine the effect of basis set on the predicted chemical shifts for benzene. Compute the NMR properties for both compounds at the B3LYP/6-31G(d) geometries we computed previously. Use the HF method for your NMR calculations, with whatever form(s) of the 6-31G basis set you deem appropriate. Compare your results to those of the HF/6-311+G(2d,p) job we ran in the earlier exercise. How does the basis set effect the accuracy of the computed chemical shift for benzene ... 

Seluflon Clearly, a hybrid functional is the best choice for this problem. We ran B3LYP calculations using the 6-31G(d), 6-31-i-G(d) and 6-311G(2d) basis sets. Here are the results ... 

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). 

If you can afford it, use B3LYP/6-31G(d) for geometries and zero-poin corrections and B3LYP with the largest practical basis set for energ calculations. 

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cyclohexane (e=2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (flfl=3.65) and using the IPCM model at the B3LYP level. The 6-31+G(d) basis set was used for all jobs. We also ran gas phase calculations for both conformations at the same model chemistries, and an IPCM calculation for the trans conformation (SCRF=Dipole calculations are not necessary for the trans conformation since it has no dipole moment). 

Basis SVWN BLYP BPW91 B3LYP B3PW91... 

As was pointed out earlier (76AHCS1, p. 217), tautomeric equilibria for substituted isoindole-isoindolenine systems depend critically upon the substituents. Isoindole exists in the o-quinoid form 6. Computational results for the parent systems are given in Table III (99UP1). The results indicate that within the B3LYP functional only large basis sets provide reliable energy differences. 

Calculated Energies for o-Tetronic Aqd (B3LYP/Basis)" (99UP1)... 

B3LYP, MP2) [00JPC(A)2599]. On the basis of these calculations, complete assignment of the vibrational spectra is provided. 

Further studies by Garcia, Mayoral et al. [10b] also included DFT calculations for the BF3-catalyzed reaction of acrolein with butadiene and it was found that the B3LYP transition state also gave the [4+2] cycloadduct, as happens for the MP2 calculations. The calculated activation energy for lowest transition-state energy was between 7.3 and 11.2 kcal mol depending on the basis set used. These values compare well with the activation enthalpies experimentally determined for the reaction of butadiene with methyl acrylate catalyzed by AIGI3 [4 a, 10]. 

The ab initio methods used by most investigators include Hartree-Fock (FFF) and Density Functional Theory (DFT) [6, 7]. An ab initio method typically uses one of many basis sets for the solution of a particular problem. These basis sets are discussed in considerable detail in references [1] and [8]. DFT is based on the proof that the ground state electronic energy is determined completely by the electron density [9]. Thus, there is a direct relationship between electron density and the energy of a system. DFT calculations are extremely popular, as they provide reliable molecular structures and are considerably faster than FFF methods where correlation corrections (MP2) are included. Although intermolecular interactions in ion-pairs are dominated by dispersion interactions, DFT (B3LYP) theory lacks this term [10-14]. FFowever, DFT theory is quite successful in representing molecular structure, which is usually a primary concern. 

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