Two approaches for the determination of the primary and secondary geometric isotope effect are compared for the exemplary porphyrinoid system porphycene, which has two intramolecular hydrogen bonds. A three-dimensional Born–Oppenheimer potential energy surface is calculated in terms of the symmetric and antisymmetric N–H stretching as well as a low-frequency hydrogen bond vibrational normal mode coordinate. From the respective ground-state nuclear wavefunction the quantum correction to the classical equilibrium geometry is determined. Further, geometry optimization within a full-dimensional multi-component molecular orbital (MC_MO) type calculation, which treats both the electrons and the hydrogen-bonded protons quantum mechanically, is performed. Both approaches yield geometric isotope effects, that is, upon H/D double substitution the hydrogen bonds are weakened and the respective N–N distances increase. In addition the MC_MO calculation gives a H/D isotope effect on the electronic structure, that is, the electronic wavefunction becomes more localized at the deuterium nucleus as compared with the proton case.