Among other inaccuracies, puckering of small fused heterocycles is of concern where MM structures keep them flat while the QM expects them to be puckered. Incorrect descriptions of puckered rings as planar geometries would result in erroneous intramolecular and intermolecular nonbonded interactions, especially in hydrogen bonding interactions and 𝜋-stacked configurations. One such deviation is shown in Figure :