The geometric structures of the precursors C 18 -(CO) n (n = 6, 4, and 2) obtained at the B97XD/def2-TZVP level are shown in Fig. , and the corresponding Cartesian coordinates are listed in Table . The optimized structures of all precursor molecules are found to be strictly planar with no imaginary frequencies, and the point group symmetries of C 18 -(CO) n with n = 6, 4, and 2 are D 3h , C 2v , and C 2v , respectively. Fig. summarizes the calculated structural parameters and available crystal data of C 18 -(CO) 6 molecule. The maximum absolute deviations of bond lengths and bond angles between theoretical calculation and experimental measurement are only 0.03 Å and -2.3º , respectively, which shows that our calculations accurately reproduced the crystal structure of the C 18 -(CO) 6 . This also emphasizes that the exchange-correlation functional of B97XD is a reliable choice for the studying of C 18 and its substitutes.