MD-determined continuum models for simulating extreme thermomechanical events in HMX explosive: How are we doing, how do we know, and how good is good enough?

I will first briefly summarize the so-called Modified Johnson-Cook (M-JC) continuum material model for mesoscale simulations of HMX under extreme conditions and then attempt to confront the three questions posed in the title. The M-JC model builds from a standard Johnson-Cook form for material strength that, for explosives such as HMX, is typically calibrated empirically—e.g., by fitting to wave-profile data from macroscale flyer-plate shock experiments—and often with limited treatment of the underlying ‘sub-grid’ physics. The M-JC description provides much of (well, some of) the missing physics that is most relevant to shock initiation, using fitted model forms wherein (i) all parameters are determined from MD and (ii) all the MD is performed using the same non-reactive force field. Continuum results for shock-induced pore collapse based on the M-JC model are in “good” agreement with ground-truth MD results, at a level that enables a meaningful discussion of the three questions at hand.