Theoretical prediction of the heats of formation, densities and relative sensitivities, and/or synthetic approaches toward the synthesis of high energy dense materials (HEDMs) : 3,5-dinitro-1,3,5-oxadiazinane, bis-adjacent RDX, bis-adjacent HMX, 4,4',6,6'-tetranitro-1,1'-bis(n-oxide)-5,5',6,6'-4H,4'H-5,5'-bisimidazo oxadiazole, and the open-cage derivative of CL-20 /