The 2nd order approach describes the shifts correctly up to order B 2 0 . Therefore, the leading order term in the difference of 2nd order and finite field shifts is a B 4 0 term. One can see that at 1.2 GHz (corresponding to the largest commercially available field strength), the relative absolute deviation is still on the order of 10 -5 , which for the largest shifts in NiSAL of around 400 ppm corresponds to around 4 × 10 -3 ppm. This deviation is in principle measurable in high-resolution NMR spectrometers but still much smaller than the 2nd order field-dependent shift, which has a size of around 1 ppm for the most shielded and deshielded protons in NiSAL-HDPT (see below) and is also much smaller than the width of the paramagnetically broadened peaks. This is evidence that the 2nd order approach, which has a couple of advantageous properties as summarized in Table , is sufficient at room temperature for the field strengths that can nowadays be routinely achieved. The left side of Figure shows the Figure : Left: Convergence of the finite field approach with respect to the size of the Lebedev grid. Plotted is the relative absolute deviation with respect to the result obtained with the largest grid. The solid line is the mean over all protons, whereas the shaded area is the region between the smallest and largest value among all protons. Right: Relative absolute deviation of shifts obtained in the 2nd order approach from shifts obtained in the finite field approach as a function of the field strength (measured in proton Larmor frequency). Below around 100 MHz, the deviation between the two approaches becomes smaller than the error introduced by the finite grid, which is why we do not show this region.