A high-performance spin simulation library Spintrum is employed to simulate zero-field NMR spectra through a numerical diagonalization of a density matrix describing the spin system . The J-coupling values in the simulations are taken directly from the literature or estimated using the gyromagnetic ratios of nuclei (see discussion above). Simulations of chemical-exchange effects on the zero-field spectra of urea were obtained using an approach presented in Ref. . Details of the calculations as well as a discussion of possible shortcomings of the used exchange model are discussed in SI.