Why are the calculated NMR values on the DFT & GIAO level of cyclopentane-1,3-dione so far off?
Brief Summary

The computational results given below indicate that there is something inherently problematic with this seemingly simple system.

Computational Methodology

Full optimizations and corresponding harmonic vibrational frequency computations of cyclopentane-1,3-dione was were at two levels of theory:


B3LYP/6-31+G(d,p)
MP2/aug-cc-pVTZ


Each optimized geometry was subjected to GIAO-NMR computations at the following levels of theory:


mPW1PW91/6-311+G(2d,p)
mPW1PW91/IGLO-II
PBE0/IGLO-II
mPW1PW91-SMD/6-311+G(2d,p)
mPW1PW91-SMD/IGLO-II
PBE0-SMD/IGLO-II


The NMR levels of theory were chosen specifically to look at basis set effects as well as implicit solvation (SMD) effects.  The solvent chosen was chloroform.

All computations were carried out using the Gaussian&nbsp;09 software package.  All DFT computations employed an ultrafine integration grid.  The frozen-core approximation was invoked for the MP2 computations.

Results

Table 1 presents the absolute isotropic shielding constants ($\sigma\ce{C}$) as well as the computed chemical shifts ($\delta\ce{C}$; determined using Equation 1 given by Martin in the comments) with all values given in ppm.  The carbons are labeled in figure 1 for cyclopentane-1,3-dione.  Despite the level of theory used for the optimization  or NMR computation, the $\delta\mathrm{C}^2$ values are consistently off by &approx;50&nbsp;ppm with respect to the 'experimental' values obtained from equation 1.



Figure 1: Cyclopentane-1,3-dione with labeled carbons



Equation 1: 
$$\delta=\frac{\text{intersection}-\sigma}{-\text{slope}}= \frac{186.5242-\sigma}{1.0533}$$

I am actually quite surprised by these results as I did not expect this system to be difficult to characterize with straightforward NMR computations.