The optimized structures of the trimers composed of one C60 fullerene and two cyclocarbons are shown in Fig. . The IGMH g inter isosurfaces represented in blue and green reveal the main interaction regions between monomers, and the angle between the fitting planes of the two cyclocarbons is also shown. It can be seen that in all trimers, due to π-π interactions, the two cyclocarbons bind to each other and are always closely attached to the C60 fullerene. As the size of the cyclocarbons increases, the angle between the two rings decreases gradually, and at the same time, C60 fullerene is more and more completely wrapped by the cyclocarbons. When the size of the cyclocarbon reaches C34, the two rings become perfectly parallel to each other. In this case, as emphasized by the blue isosurface in Fig. (e), a clear circular - stacking region is formed between the two C34, and the fullerene is embedded precisely at its center. Incidentally, in 2C34@C60, the shorter C-C bond of one cyclocarbon faces the longer C-C bond of another one, as observed in our study of - stacked C18 dimer, and this phenomenon can be well explained by the electrostatic potential complementary rule. Fig. Optimized geometry of C60@2Cn trimers (n = 18, 22, 26, 30, and 34). Isosurfaces of g inter function with isovalue of 0.003 a.u. are shown to reveal main interaction regions between two cyclocarbons (colored by blue) and between cyclocarbons and fullerene (colored by green). The angles between the fitting planes of the two cyclocarbons are also labelled. ) and for C60@2Cn trimer formation between C60@Cn and another Cn ( that the later added cyclocarbon bind more easily than the first one, and this trend is more pronounced for large-sized cyclocarbons. This phenomenon shows an evident synergistic effect brought by intermolecular interaction between cyclocarbons. We further characterized the interaction strength between the constituent monomers in the C60@2Cn trimer using interaction energy