Gauge invariance is tested by selecting the CBS(TQ) extrapolation with LG(OI) as reference for all choices of gauge within a method, and the results are shown in Figure . This choice of reference is somewhat arbitrary, in the sense that any of the gauges could be used. However, LG(OI) was selected because of its slightly faster convergence rate towards the CBS limit compared to the other choices of gauge. Since gauge invariance is expected for variational methods at the CBS limit, it is worth discussing the DFT and CCSD results separately. For the functionals in panels It is apparent from Figure (c) that CCSD does not approach gauge invariance at the CBS limit (the results from Figure indicate that the CBS(TQ) extrapolation is sufficiently close to the CBS limit). Gauge invariance is not expected for standard approximate CC methods unless the treatment of electron correlation is complete, as discussed in section 2 and demonstrated numerically in Ref. 40. Therefore, the difference between choices of gauge at the CBS limit is a measure of the electron correlation incompleteness for a particular level of truncation in the excitation expansion. A value of ∆ TQ/LG(OI) U = 0.26 for the CBS(TQ) extrapolation with the MVG indicates that CCSD is in fact still quite far from convergence in the treatment of electron correlation (at least for this test set). The ∆ TQ/LG(OI) U change for LG by a factor of 3 (from 0.44 with aug-cc-pVDZ to 0.13 at the CBS limit) indicates a considerable decrease in the "origin dependence" for this choice of gauge. In comparison, the corresponding change for MVG is only by a factor of 2 (from 0.59 to 0.26). It can also be inferred from the graph that the MVG tends to give OR values that are larger in magnitude relative to LG(OI), as evidenced by the positive values of ∆ TQ/LG(OI) S from MVG across all basis set sizes. The opposite can be said for LG, which shows a tendency to underestimate the LG(OI) reference OR.