Atomistic simulations were used to calculate the isothermal elastic properties for b-, a-, and d-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine ~HMX!. The room-temperature isotherm for each polymorph was computed in the pressure interval 0<p<10.6 GPa and was used to extract the initial isothermal bulk modulus Ko and its pressure derivative using equations of state employed previously in exper...