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Chemical Shifts

NMR shifts are obtained by comparing nuclear shieldings of your test compound with a reference molecule ( $ \delta_{subst} = \delta_{ref} +
\sigma_{ref} - \sigma_{subst}$). Therefore you have to choose a reference molecule with a well-known shift for which you can easily calculate the absolute shielding constant. This implies a certainty about the geometry, too. Furthermore you have to use the very same basis set for corresponding atoms to minimize the basis set influence.