The geometric structures and excitation characteristics of the C 18 -(CO) n (n = 2, 4, and 6) were respectively obtained by DFT and time-dependent DFT (TD-DFT) calculations at ωB97XD [29]/def2-TZVP level in the gas phase. Our recent theoretical work has demonstrated that this computational strategy can reproduce the crystal structure of C 18 -(CO) 6 well, so it is considered to be competent to describe the present systems . The molecular (hyper)polarizabilities were calculated by both analytic derivatives of the system energy [namely, coupled-perturbed Kohn-Sham (CPKS)] and finite field (FF) methods at the ωB97XD/aug-cc-pVTZ(-f,-d) level, where aug-cc-pVTZ(-f,-d) is a reduced version of the aug-cc-pVTZ basis set with the removal of f-type polarization functions of non-hydrogen atoms and d-type polarization functions of hydrogen atoms. The (hyper)polarizability densities were evaluated at the same level. For detailed formulas for calculating molecular response properties, please refer to Ref. . Gaussian 16 (A.03) program package was utilized for all quantum chemistry calculations . Multiwfn 3.8(dev) code was employed to perform the electronic structure analyses and plot charge-transfer spectrum (CTS) based on the output and formatted check point files of Gaussian 16 . The unit sphere representation and local contribution map of (hyper)polarizabilities were rendered by means of Visual Molecular Dynamics (VMD) software based on the files exported from Multiwfn 3.8(dev). The colored contour maps of local contribution of (hyper)polarizabilities were plotted directly via Multiwfn 3.8(dev).