Basis sets cc-pVDZ

Basis set cc-pVDZ unless otherwise indicated geometries from BPW91/cc-pVDZ density functional calculations (see Chapter 8). 

Two tailored basis sets were used in [42] A 4-31G- -pd basis set based upon a standard 4-3IG basis set augmented with diffuse p(0.0780) and d(0.1870) exponents. The other basis set cc-pVDZ- -spd is based upon the standard cc-pVDZ basis set, augmented with s(0.0469), p(0.0800) and d(0.3140) exponents. Experimental data for the polarizability a, which have been deduced from the measurements of the dielectric constant in thin films, are in the range of 579-595 a.u. [85-87]. Another... 

In the case of C2 we see no singularity on the negative real axis. This may be due to the fact that the basis set (cc-pVDZ) lacks diffuse functions. 

An informative study on excited states of diatomic molecules has been made by Sattelmeyer et al. [69], This study includes a comparison with FCI results as well as a comparison of some extended basis set CC values of re, coe, and Te with experimental data. A total of 7 valence excited states were studied BH (1n) CH+ (1I1) C2 ( u and 1FLU) CO (1I1) N2 Ilg and 1 ). First, for BH and CH+ CCSD, CC3, CCSDT-3, CCSDT, CCSDTQ, and FCI results were obtained with the cc-pVDZ basis set. Next, all molecules were studied with the CCSD, CC3, and CCSDT-3 methods and the cc-pVDZ, cc-pVTZ, cc-pVQZ, and cc-pV5Z basis sets. Diffuse functions were not included since the excited states considered are of valence character. 

CO Peterson and Dunning [92] have made an extensive analysis of the role of basis sets and correlation treatments in the calculation of the molecular properties of CO. By carefully controlling the errors in the calculations, it was possible to compute properties of this small molecule to an accuracy that rivals the most sophisticated experimental studies. They made use of the correlation consistent basis sets (cc). The dissociation energy with icCAS+SDQ was computed 258.5 kcal/mol with the best method, and the experimental value is 259.6 0.1 kcal/mol. The CCSD(T) yielded 258.6 kcal/mol in excellent agreement with experiment. CASSCF, MP4, and CCSD yield results with errors bigger than 4 kcal/mol. The CBS limit was obtained by exponential extrapolation of the cc-pVDZ through cc-pV6Z for all methods. 

From Figure 15.8, we see that the calculated dipole moments are more sensitive to the choice of the A -electron model than to the choice of cardinal number for the basis set. Even in the smallest basis, the CCSD(T) model is considerably more accurate than the CCSD model. Thus, for the CCSD(T) model, the smallest basis (aug-cc-pVDZ) gives a mean error of —0.006 D and the largest basis (aug-cc-pVQZ) a mean error of -1-0.006 D, the corresponding mean absolute errors being 0.02 and 0.008 D. In comparison, for the CCSD model, the mean errors are 0.02 and 0.03 D at the aug-cc-pVDZ and aug-cc-pVQZ levels, respectively, and the mean absolute errors 0.04 D in both cases. Whereas, at the CCSD and CCSD(T) levels, the dipole moment increases with the cardinal number, at the Hartree-Fock level, the change is in the opposite direction. The performance of the MP2 model appears to be less systematic. 

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. 

Figure 11.1 shows the bond dissociation curve at the HF level with the STO-3G, 3-21G, 6-31G(d,p), cc-pVDZ and cc-pVQZ basis sets. The total energy drops considerably upon going from the STO-3G to the 3-21G and again to the 6-3IG(d,p) basis. This is primarily due to the improved description of the oxygen Is-orbital. The two different... 

Molecular geometries, which were not reported in this study, have been obtained using the 6-31 G(d,p) and cc-pVDZ basis sets. Thus, the 6-31 lG(d,p) and cc-pVTZ results refer to single point energy calculations only. 

---