Very large trimeric indole alkaloid strychnohexamine, with empirical formula C59H60N6O (66 second-row atoms and 60 protons), has been subjected to the state-of-the-art computation of the 1H and 13C nuclear magnetic resonance (NMR) chemical shifts of its configurational isomers at each of the 14 asymmetric centers. Several spectral reassignments and corrections of 1H and 13C NMR spectra of this alkaloid were suggested based on the PBE0/pcSseg-2//pcseg-2 calculation of its NMR chemical shifts. Thus, all pairs of diastereotopic protons were assigned together with four aromatic carbon resonances of C-9 and C-11, C-9″, and C-11″. In addition, the unassigned chemical shifts of carbon C-23″ and proton at C-3′ in, accordingly, 13C and 1H NMR spectra were predicted. © 2021 John Wiley & Sons, Ltd.