Chemical Shifts

NMR shifts are obtained by comparing nuclear shieldings of your test compound with a reference molecule ( δ_subst = δ_ref + σ_ref - σ_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.