Cc-pVTZ

CCSD(T)/aug-cc-pVTZ Atomization energy 5 kcal/mol mean abs. dev. 

Electron correlation studies demand basis sets that are capable of very high accuracy, and the 6-31IG set I used for the examples above is not truly adequate. A number of basis sets have been carefully designed for correlation studies, for example the correlation consistent basis sets of Dunning. These go by the acronyms cc-pVDZ, cc-pVTZ, cc-pVQZ, cc-pV5Z and cc-pV6Z (double, triple, quadruple, quintuple and sextuple-zeta respectively). They include polarization functions by definition, and (for example) the cc-pV6Z set consists of 8. 6p, 4d, 3f, 2g and Ih basis functions. 

I have included some representative results in the table, using the cc-pVTZ basis set. 

CASSCF(2,2)/cc-pVTZ Symmetry=Loose Guess=Read Geom=Check... 

A+B L -fl/2) have also been used. The theoretical assumption underlying an inverse power dependence is that the basis set is saturated in the radial part (e.g. the cc-pVTZ ba.sis is complete in the s-, p-, d- and f-function spaces). This is not the case for the correlation consistent basis sets, even for the cc-pV6Z basis the errors due to insuficient numbers of s- to i-functions is comparable to that from neglect of functions with angular moment higher than i-functions. 

A comparison between Gl, G2, G2(MP2) and G2(MP2,SVP) is shown in Table 5.2 for the reference G2 data set the mean absolute deviations in kcal/mol vary from 1.1 to 1.6 kcal/mol. There are other variations of tlie G2 metliods in use, for example involving DFT metliods for geometry optimization and frequency calculation or CCSD(T) instead of QCISD(T), with slightly varying performance and computational cost. The errors with the G2 method are comparable to those obtained directly from calculations at the CCSD(T)/cc-pVTZ level, at a significantly lower computational cost. ... 

Method cc-pVDZ cc-pVTZ cc-pVQZ cc-pV5Z cc-pV6Z cc-pVooZ... 

We will now look at how different types of wave functions behave when the O-H bond is stretched. The basis set used in all cases is the aug-cc-pVTZ, and the reference curve is taken as the [8, 8J-CASSCF result, which is slightly larger than a full-valence Cl. As mentioned in Section 4.6, this allows a correct dissociation, and since all the valence electrons are correlated, it will generate a curve close to the full Cl limit. The bond dissociation energy calculated at this level is 122.1 kcaPmol, which is comparable to the experimental value of 125.9 kcal/mol. 

The HF method overestimates the barrier for linearity by 0.73 kcal/mol, while MP2 underestimates it by 0.76 kcal/mol. Furthermore, the HF curve increase slightly too steeply for small bond angles. The MP4 result, however, is within a few tenths of a kcal/ mol of the exact result over the whole curve. Compared to the bond dissociation discussed above, it is clear that relative energies of conformations which have similar bonding are fairly easy to calculate. While the HF and MP4 total energies with the aug-cc-pVTZ basis are 260 kcal/mol and 85 kcal/mol higher than the exact values at the equilibrium geometry (Table 11.8), these errors are essentially constant over the whole surface. 

---