The absorption properties of the pristine and functionalized adamantane structures were calculated using the TD-B3LYP/aug-cc-pVDZ method and summarized in table 3. It is clear that the excitation energy for the pristine adamantane is the highest, but for all the functionalized adamantane molecules, they are too low ranging from 0.5 to 2.0 eV. Furthermore, these energies are dramatically reduced for the adamantane with one Li/Na atom substitution and slightly decreasing for adding more Li/Na atoms. According to the greatest oscillator strength (f), the optical gap can be defined as the energy difference between the ground state (S0) and the first exited state (S1). It can be clearly observed from table 3 that the calculated optical gap of the pristine adamantane is 6.417 eV, which is good agreement with the experimental value of 6.49 eV [7] having the relative error of 1.12 %. The optical gaps for all the functionalized adamantane molecules show similar trends as the calculated energy gap in table 1. Figure 1 shows the absorption spectrum as a function of excitation energy for the pristine and the functionalized adamantane molecules.