Sand has many avatars – it can behave as a solid, liquid or gas, depending on external circumstances. This multiple identity is one of several reasons why the computer simulation of dry granular materials is difficult. Sand in the solid-like state responds to external stimuli on a very different timescale to sand in its liquidlike avatar – in contrast to most efficient computer simulation methods, which are typically tuned to one particular timescale such as a collision or relaxation time. Other features of sand which are difficult to simulate efficiently include complex, dissipative interparticle and particle–wall interactions, typically irregular grain shapes and strong hysteretic effects. Furthermore, the athermal nature of sand means that grains do not randomly sample all possible states ergodically – as a result, appropriate statistical averages can only be obtained by repeated (computationally demanding) simulations of a granular system.

For normal dry powders, interstitial fluid plays only a minor role – apart from exceptional cases when, say, there are small liquid pools at particle contacts which could seriously alter the pairwise nature of grain interactions. This is a clear distinction between granular systems and dense suspensions – for the former, interparticle interactions are restricted to short-ranged contact forces. In practice, the methods developed for granular simulations are quite similar to classical methods used to simulate simple liquids.