Values obtained within the 6-311G++(2d,2p) and 6-311G(d,p) basis sets demonstrate good agreement, therefore, the smaller basis set is found to be sufficient for the CL-20 and its derivatives. CL-20 and CSi5H6N12O12 cages have almost the same energies of frontier orbitals and, therefore, possess rather similar chemical properties and reactivity. The presence of silicon atoms in the molecular framework does not change significantly the values of HOMO and LUMO energies since LUMO orbitals mainly concentrate on the nitro groups (see Fig. 4).