Atomic calculations, convergence with correlation consistent basis

Convergence of Atomic Calculations with Correlation Consistent Basis Sets... 

The experience gained in these studies has been invaluable for the development of the BERTHA molecular code much of the material of the present article was first presented in [29] and [30], together with applications to the study of magnetic and hyperfine interactions in atoms and small molecules, NMR shielding constants for H2O and NH3, and P-odd interactions in chiral molecules such as CHBrClF. A detailed study of the water molecule [31] examined the convergence of the DHF and DHFB calculations with a series of uncontracted correlation consistent basis sets due... 

The augmented correlation consistent basis sets, when combined with coupled cluster or multireference configuration interaction methods, have been found to provide an accurate description of atomic electron affinities. The calculated EAs of boron and fluorine are summarized in Table 26 and the basis set convergence errors are plotted in Figure 13. The values of the EAs of these two atoms, 6.39 0.23 kcal mol (B) and 78.82 kcal mol" (F), respectively (corrected for spin-orbit effects), bracket the EAs of the other first-row atoms. 

The first point to note about the correlation-consistent basis sets in Table 8.16 is that the convergence is in all cases uniform and systematic - for the energies, for the bond distances, and for the bond angle. Scrutiny of the table reveals that, with each increment in the cardinal number, all errors are reduced by a factor of at least 3 or 4. Clearly, the correlation-consistent basis sets provide a convenient framework for the quantitative study of molecular systems at the Hartree-Fock level. We also note that the results for the cc-pVXZ and cc-pCVXZ basis sets are very similar. Apparently, the molecular core orbitals are quite atom-like and unpolarized by chemical bonding. In Hartree-Fock calculations, therefore, the use of the smaller valence cc-pVXZ sets is recommended. 

Dunning has developed a series of correlation-consistent polarized valence n-zeta basis sets (denoted cc-pVnZ ) in which polarization functions are systematically added to all atoms with each increase in n. (Corresponding diffuse sets are also added for each n if the prefix aug- is included.) These sets are optimized for use in correlated calculations and are chosen to insure a smooth and rapid (exponential-like) convergence pattern with increasing n. For example, the keyword label aug-cc-pVDZ denotes a valence double-zeta set with polarization and diffuse functions on all atoms (approximately equivalent to the 6-311++G set), whereas aug-cc-pVQZ is the corresponding quadruple-zeta basis which includes (3d2flg,2pld) polarization sets. 

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