DIFFREALWAVE: A parallel real wavepacket code for the quantum mechanical calculation of reactive state-to-state differential cross sections in atom plus diatom collisions A parallel computer code for the calculation of quantum state-to-state atom-diatom differential reactive cross sections is presented and discussed. The code is based on the real wavepacket approach. The theory underlying the code is discussed and the parallelisation methods used are described. All the input parameters needed by the program are described. Results of test calculations to investigate the scaling properties of the code with grid size and number of processors are presented.