Crystalline and amorphous diazepam, a psychoactive drug, were investigated by employing spin-lattice relaxation ¹H NMR along with atom–atom calculations of the landscape of energy barriers. The activation barriers for reorientation of the methyl group in amorphous diazepam were found to be in the range of 1.9–12.7 kJ/mol. Atom–atom calculations permitted determination of the distribution of energy barriers for reorientations of methyl groups, which was in a good agreement with that obtained on the basis of experimental data. The NMR relaxation combined with calculations provided a quantitative description of the distribution of energy barriers including intra- and inter-molecular interactions