Disorder in caffeine (XVa): (a) An ordered representation of the structure of form II caffeine (Enright et al., 2007) with the ordered molecule in atomic colouring, and the two independent disordered components in red and blue. The stacking was used to classify the structures on the crystal energy landscape in Fig. 1(c) (i.e. stack 1 atomic, stack 2 red, stack 3 blue). (b) The electrostatic potential on the van der Waals surface plus 1.2 Å for caffeine; +1.5 V corresponds to an interaction energy of +1.4 kJ mol−1 with a positive point charge of 0.01 e. The experimental structure has disorder components corresponding to rotation by 180° about the two marked axes (Habgood, 2011), with the heavier dashed line corresponding to the blue disorder component which has greater approximate symmetry in the steric and electrostatic interactions of the molecule than the red component which corresponds to a crystallographic axis.