There is this method called nucleus-independent chemical shift (NICS) to calculate the magnetic shielding values at different spatial coordinates of a molecular system. It can be used to estimate the aromatic character of a system by evaluating the magnetic shieldings inside a potentially aromatic cyclic system. The calculated values correspond to the magnetic shielding tensor, the elements of which are given by:
$\sigma = \frac{\partial^{2} E}{\partial B_{i} \partial m_{j}}$
Furthermore there is the method of gauge-including magnetically induced currents (GIMIC) which can also be used to estimate the aromaticity of a system based on ring currents of a conjugated cyclic system. In this case the current density susceptibility tensor is evaluated.
Because the quantities studied by the aforementioned methods arise from the influence of external magnetic fields I am wondering how high the external magnetic field implied in these calculations actually is. Is there even an external magnetic field implied in these calculations or am I missing something here ? I know that chemical shifts are basically independent of the external magnetic field but only because of the use of an NMR standard, which eliminates this dependency. The answer seems simple but I am kind of caught in confusion by now.