Problematic Systems

The H2O case is an example of a system where it is relatively easy to obtain good results. Nature is not always so kind let us look at a couple of theoretically difficult cases. 

The H2O case is an example of a system where it is relatively easy to obtain good 

The FOOT molecule has an experimental geometry with an O—O bond length of 1.217 A and an F—O bond of 1.575 A. The calculated bond distances at different levels of theory with the aug-cc-pVTZ basis set are given in Table 11.20. 

The results in Table 11.20 clearly show that the results are very sensitive to the inclusion of electron correlation. The MP4(SDQ) geometry is very similar to the CCSD one 

The DFT methods are all well behaved and perform surprisingly well for such a difficult system, with the B3LYP results being comparable to CCSD. The main problem is of course that there is no way of systematically improving the structure, or knowing beforehand whether DFT will be able to give a good description for the specific problem. 

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For problematic systems we also implemented the damped Coulomb operator 0 = erfc fir)/r proposed by (Jung et al., 2005 to attenuate the near singular behavior due to long range interactions present in G. In our studies we observed that noise is still a problem for some systems. The noise in the fit is known to arise due to the attempt of the ABSs to fit the density at the nuclear cores (Cisneros et al., 2005b, 2006a. ... 

Another problematic system for the ALDA is the streched H2 molecule [61,62]. Prom a comparison with exact results it was found that the ALDA fails to reproduce even qualitatively the shape of the potential curves for the and states. A detailed analysis of the problem shows that the failure is related to the breakdown of the simple local approximation to the kernel. 

DFT Methods Angle Bending Curve Problematic Systems... 

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