Previous theoretical studies on the molecular structure of C 18 have demonstrated that only the exchange-correlation functionals with more than 25% Hartree-Fock exchange can reliably exhibit the ground-state geometry of the molecule , and the B97XD/def2-TZVP level has been proved to robustly reproduce the high-precision computational result and experimental observation. Hence, we still adopt the same strategy as our previous works on C 18 to optimize the geometry of precursors C 18 -(CO) n (n = 6, 4, and 2).