I wanna model the proppant placement in the hydraulic fracture using molecular dynamics, but I am the beginner of MD, I read some articles about the modelled shape of research object (mainly, parallel plate, or rectangular object ), by contrast, the shape of sphere is relatively spare. Besides, I wanna to model large particles, the size of proppant is usually between 0.5-1 mm, how to realise this condition (using coarse-grained MD)?