DL_POLY_3: the CCP5 national UK code for molecular-dynamics simulations. DL_POLY_3 is a general-purpose molecular-dynamics simulation package embedding a highly efficient domain decomposition (DD) parallelization strategy. It was developed at Daresbury Laboratory under the auspices of the Engineering and Physical Sciences Research Council. Written to support academic research, it has a wide range of applications and will run on a wide range of computers; from single-processor workstations to multi-processor computers, with accent on the efficient use of multi-processor power. A new DD adaptation of the smoothed particle mesh Ewald method for calculating long-range forces in molecular simulations, incorporating a novel three-dimensional fast Fourier transform (the Daresbury Advanced Fourier Transform), makes it possible to simulate systems of the order of one million particles and beyond. DL_POLY_3 structure, functionality, performance and availability are described in this feature paper.