| 4:45:35 PM PST - Thu, Dec 10th 2015 |
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The other aspect to consider is that the long range behavior of most density functionals (especially pure DFT ones like LDA) is incorrect and decays too quickly. For instance, just using HF exchange with this same set up (H2 molecules 3 Angstrom apart) with the 3-21G basis set results in complete oscillation of the charge as opposed to the partial oscillation that LDA gives.
In general you want to use the largest basis set that you can afford; although, cancellation of errors can result in a smaller basis set giving numbers that agree better with a reference than a larger basis set.
Some systems just have a strong dependence on the basis set.
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