  We present a new algorithm, MULTIGRAIN, for modelling the dynamics of an
entire population of small dust grains immersed in gas, typical of conditions
that are found in molecular clouds and protoplanetary discs. The MULTIGRAIN
method is more accurate than single-phase simulations because the gas
experiences a backreaction from each dust phase and communicates this change to
the other phases, thereby indirectly coupling the dust phases together. The
MULTIGRAIN method is fast, explicit and low storage, requiring only an array of
dust fractions and their derivatives defined for each resolution element.
