Basis sets geometry convergence

We need to look at the convergence as a function of basis set and amount of electron correlation (Figure 4.2). For the former we will use the correlation consistent basis sets of double, triple, quadruple, quintuple and, when possible, sextuple quality (Section 5.4.5), while the sensitivity to electron correlation will be sampled by the HF, MP2 and CCSD(T) methods (Sections 3.2, 4.8 and 4.9). Table 11.1 shows how the geometry changes as a function of basis set at the HF level of theory. 

Martin, J. M. L., El-Yazal, J., Francois, J.-P, 1995a, Basis Set Convergence and Performance of Density Functional Theory Including Exact Exchange Contributions for Geometries and Harmonic Frequencies , Mol. Phys., 86, 1437. 

As in the case of ArHF, the uncorrected first increases and then decreases when the basis set is improved from aug-cc-pVDZ to aug-cc-pVQZ. The corrected results converge much more regularly. Similar patterns have been observed for other systems as well, including FfH O) and ci (H20) (Xantheas, 1996). The posthoc counterpoise-corrected D s are very close to the results obtained by optimizing the dimer geometry on the counterpoise-corrected surface only for the aug-cc-pVDZ set is the result significantly different. This is due to the fact that the uncorrected F-F distances are too short, an effect that is largest for the aug-cc-pVDZ set. 

To expand a bit on Dewar s cautious endorsement of the SCF procedure [20] ( SCF calculations are by no means foolproof . ..Usually one finds a reasonably rapid convergence to the required solution ) occasionally a wavefunction is obtained that is not the best one available from the chosen basis set. This phenomenon is called wavefunction instability. To see how this could happen note that the SCF method is an optimization procedure somewhat analogous to geometry optimization (Section 2.4). In geometry optimization we seek a relative minimum or a transition state on a hypersurface in a mathematical energy versus nuclear coordinates space defined by E =/(nuclear coordinates) in wavefunction... 

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