6-311G basis set

In order to describe the number of primitives and contractions more directly, the notation (6s,5p) (ls,3p) or (6s,5p)/(ls,3p) is sometimes used. This example indicates that six s primitives and hve p primitives are contracted into one s contraction and three p contractions. Thus, this might be a description of the 6—311G basis set. However, this notation is not precise enough to tell whether the three p contractions consist of three, one, and one primitives or two, two, and one primitives. The notation (6,311) or (6,221) is used to distinguish these cases. Some authors use round parentheses ( ) to denote the number of primitives and square brackets [ ] to denote the number of contractions. 

IG(d) arbitrarily assigns an exponent of 0.8 to all four elements. The dagger basis set uses the exponents from the d functions in the 6-311G basis set (you can run an additional job to verify this), which have been individually optimized for each element. 

Figure 3.12. Optimized structure of the free triplet state (a) and the triplet of the carbonyl H-bond complex (b) calculated from the DPT calculations using the UB3LYP method with a 6-311G basis set. (Reprinted with permission from reference [42]. Copyright (2005) American Chemical Society.)...

Highly energetic compounds with potential use in explosive devices must be characterized completely and safely, particularly as the explosive character may be linked directly to vibrational modes in the molecular structure, hence the application of computational methods to complement experimental observations. ANTA 5 has been the subject of various studies and, as an adjunct to one of these and to confirm the results of an inelastic neutron scattering experiment, an isolated molecule calculation was carried out using the 6-311G basis set <2005CPL(403)329>. 

The exception is the 6-311G basis set which has been formulated using MP2 calculations on atoms rather than Hartree-Fock calculations. 

In the G2 method, the 6-311G basis set and its derivatives are not defined for second-row atoms instead, a basis set optimized by McLean and Chandler (1980) is used. 

The first correction is that due to diffuse sp basis functions. These are known to be important for anions and molecules with lone pairs. The correction is obtained at MP4 level, comparing the energy of the 6-311 + G and the 6-311G basis sets. 

The third correction allows for inadequacies in the MP4 treatment. The QCISD(T) method is equivalent to MP4 in fourth order and also correctly incorporates parts of the fifth and higher orders. We therefore make a quadratic Cl correction using the 6-311G basis set. This is a very expensive calculation for molecules of medium size. 

For the Electron Localisation Function (ELF) analyses, we have used the 6-311G basis set with the PW functional to be able to include the core basins in the representation. 

As we just saw, MP2 calculations utilize the Hartree-Fock MOs (their coefficients c and energies e). The HF method gives the best occupied MOs obtainable from a given basis set and a one-determinant total wavefunction i(i, but it does not optimize the virtual orbitals (after all, in the HF procedure we start with a determinant consisting of only the occupied MOs - Sections 5.2.3.1-5.2.3.4). To get a reasonable description of the virtual orbitals and to obtain a reasonable number of them into which to promote electrons, we need a basis set that is not too small. The use of the STO-1G basis in the above example was purely illustrative the smallest basis set generally considered acceptable for correlated calculations is the 6-31G, and in fact this is perhaps the one most frequently used for MP2 calculations. The 6-311G basis set is also widely used for MP2 and MP4 calculations. Both bases... 

Jensen carried out a structural determination of l,3-dichloro-l,3-diazetidine-2,4-dione 45 <2004SAA2719>. Due to its symmetry and unusual bonding, l,3-dichloro-l,3-diazetidine-2,4-dione is an interesting case for quantum chemical analysis. The vibrational frequencies of 45 were calculated using 6-311G" basis set. The calculation utilized the Cy, symmetry of 1, 3-dichloro-l, 3-diazetidine-2, 4-dione molecule (Tables 6 and 7). 

Each vibrational mode was assigned to one of the six types of motion predicted by theoretical analysis (C=0, stretch, N-C stretch, N-Cl stretch, N-C-N bend, N-Cl bend, and C=0 bend). The vibrational frequencies of 1,3-dichloro-l,3-diazetidine-2,4-dione 45 were calculated at Hatree-Fock level, DFT (B3LYP) level, and MP-2 level of theory using a standard 6-311G basis set. For N-C stretching modes all operators except E have a trace of zero. 

Ammal et al. examined Reaction 8.13 by first locating all of the critical points on the PES. With the HE, MP2, and B3LYP methods and either the 6-31G or the 6-311G basis sets, the only TS that could be located is for the concerted process, 93 (Figure 8.20). The activation barrier is 5.5 (HF/6-31G ),... 

The last row of Table 2.3 reveals the profound difficulties in using the 6-311G basis set where the triple split of the valence set might normally be expected to be an improvement over 6-3IG. Instead, the SCF BSSE is more than doubled, and an increase of similar magnitude occurs in the correlated superposition error. Indeed, the correlation component is completely distorted by superposition effects Essentially all the (1.93 kcal/mol) stabilization predicted by MP2 with this basis set is due to the artifact of superposition. Removal of this error leaves only a net stabilization of less than 0.1 kcal/mol. 

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