6-311G is not of valence triple-zeta quality

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Abstract
We have discovered a weakness in the construction of the 6-311G basis set. The second set of three contracted s-functions is, in fact, not a valence orbital but is instead a 1s function. Thus in the s space this basis set is better represented as 63-11. Coupled with an earlier observation that additional diffuse p functions are required to yield results comparable to those of the Huzinaga–Dunning (10s6p/5s3p) triple-zeta basis set in certain cases, this suggests that greater caution should be exercised in the use of the 6-311G basis set.