B3LYP level optimizations were performed on the structures of the octasilsesquioxane (Si 8 O 12 H 8 , HT 8 ) double four-ring (D4R) cage and single hydrogen atom-trapped HT 8 (H@HT 8 ). Moreover, the transition state in the detrapping process of the hydrogen atom from the D4R cage was examined. The basis sets used were 6-31G** for HT 8 and (3 1 1/1*1*/1*) for the trapped hydrogen atom. Both HT 8 and H@HT 8 were structure-optimized with O h molecular symmetry and the resulting cage conformations were similar. The trapped H atom was located at the center of the D4R cage. The weak interaction between the D4R cage and the trapped H atom in H@HT 8 was determined by examining the singly occupied molecular orbital (SOMO) [8a 1g ] of H@HT 8 . The SOMO was constructed from an antibonding interaction between the lowest unoccupied molecular orbital (LUMO) [8a 1g ] of HT 8 and the 1s orbital of the trapped H atom. For the transition state, the structure was optimized with C 4V molecular symmetry. As a result, the position of the Si 8 cube framework was unchanged, and four O atoms in a silicon single four-ring were displaced, thereby opening one of the oxygen windows of the D4R cage. The detrapping H atom was located near the center of the oxygen window and the MO illustrations showed a change in shape from spherical to ellipsoid. Consequently, it is clear that the detrapping process is not due to the formation of chemical bonding. The calculated activation and reaction energies of this detrapping process were +98.6 and -26.1 kJ/mol, respectively. In addition, single-point calculations at the MP2 level were done for each optimized structure, and the obtained activation and reaction energies were +128.7 and -9.3 kJ/mol, respectively. Both calculated activation energies were comparable to Sto ¨sser's experimental data (+109.6 ( 3.1 kJ/mol) for H•:Si 8 O 12 (OSi(CH 3 ) 3 ) 8 (Q 8 M 8 ). Furthermore, additional explanations are given on the IR vibrational frequencies of HT 8 and H@HT 8 and the hyperfine coupling constant for caged atomic hydrogen by ESR.