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Initially , each processor generates this list for subbox pairs in which local subboxes and nonlocal subboxes in half of the directions are involved , i.e ., north , but not south , etc. Pairs of subboxes on the same processor are always handled by that processor . Each processor is small enough where adding multiple chips does n't increase the size of the phone 's motherboard dramatically , he said . At every step in the calculation , each processor needs to know what the results are in the neighboring volumes before calculating the next step . One method of doing so is to have a partitioned secondary storage in which each processor owns a logical disk . Each processor owns a range of subboxes in each of the three dimensions , i.e ., the processors are logically arranged into a three-dimensional grid . The deepest level cells are assigned to processors such that each processor has contiguous cells and static load balancing is achieved by redistributing the cells so that each processor has approximately same number of atoms . That is , for optimum efficiency you are forced to depart from the traditional von Neumann uniprocessor architecture , and instead use a parallel , multiprocessing architecture , where each processor is associated with some local memory . Note that in this figure each processor owns four subboxes , and that the cutoff radius extends two subboxes . In this way , each processor owns a subgrid of subboxes . Based on the cutoff radius and the size of the subboxes , each processor generates a pair list of subboxes for which interactions need to be evaluated . In the second class of parallelization approaches , the physical space in the molecular system is distributed instead of the atom list.8-13 In this scheme , each processor is responsible for the calculation of forces of the atoms that occupy the region or domain assigned to the processor . The subbox pair list on each processor is ordered such that atomic coordinates from a remote subbox are retrieved only once per molecular dynamics step .