In the simplest case, you may only want to see the calculated values for the magnetic shielding tensors. Choose "Just extract NMR data" and click on the GaussSum logo. The relevant information is written to the screen.

To use the NMR.py module of GaussSum most effectively, it is necessary to use a comment line of the following format: NAME(chemical name) THEORY(level of theory). For example, "NAME(benzene) THEORY(B3LYP/6-31G(d,p)//B3LYP/6-31G)" which indicates that the NMR calculation was carried out using a large basis set, after a geometry optimisation using a smaller basis set.

If you want to add a standard to nmrstandards.txt, you should carry out the NMR calculation using a comment as described in the previous paragraph. Next, using GaussSum NMR.py, choose "Take level of theory from comment" and "Add new standard". Click on the GaussSum logo to add the standard to nmrstandards.txt. For example, I optimised the structure of TMS (tetramethylsilane) under tetrahedral symmetry at the B3LYP/STO-3G level of theory, and calculated the magnetic shielding tensors at the same level.

If you want to get the relative ppm of a molecule to one of the standards (at a particular level of theory) you should either choose "Take level of theory from comment" (this is the recommended method) or "Take level of theory from listbox" (which lists the levels of theory for which there are standards available). Next choose "Calculate relative ppm" and click on the GaussSum logo.