Electron correlation error

In theory there is no limit to the number of functions that could be included in a basis set. Indeed, theoreticians often refer to the "Hartree-Fock limit as the energy that would be calculated for a system if an infinite number of basis functions were included in the calculation. That energy would still be greater than the energy of the molecular system, however, because the HF method cannot account for the additional stabilization to be gained through electron correlation. Nevertheless, HF calculations can provide very useful information if the electron correlation errors cancel when one compares one system to another. This is more likely to be the case for isodesmic reactions, in which types of bonds are the same in both the reactant and product. ... 

The results of calculations of ion-molecule binding energies are made very reliable by fortunate cancellation of the electron correlation errors (23-24). The influence of Li ", Na" ", and... 

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