The amantadine cation (Ama + ) and the cyanoadamantane cation (Ada-CN + ) have been optimized in the ground (D 0 ) state at the UB3LYP /6-31G* level of theory (Fig. ). To confirm the minimum nature of the obtained geometries, a normal mode analysis was performed for the optimized structures using the same level of theory, and no imaginary frequencies were found. Then, the vertical excitation energies of the D 0 → D i transitions and the corresponding oscillator strengths were calculated at the minimum geometries of the cations. To do so, linear response time-dependent density functional theory (TD-DFT) with two density functional approximations, namely TD-UB3LYP/6-31G* and TD-UCAM-B3LYP /6-31G*, as well as the semiempirical configuration interaction singles and doubles (CISD) method were used. Specifically, the restricted open-shell OM3 method was employed to generate molecular orbitals for CISD calculations. This method was also applied to study nonadiabatic dynamics of the adamantane cation (Ada + ) in our recent work . The CISD calculations were performed in the restricted active space of x occupied and y virtual frontier orbitals. Specifically, after test calculations of vertical spectra, we set x = 9 and y = 10 for Ama + , and x = 12 and y = 10 for Ada-CN + . The semiempirical method is thus termed ROOM3/CISD(x × y). DFT and TD-DFT calculations were performed using Gaussian 16 , and the ROOM3/CISD(x × y) calculations were done with the MNDO program .