Instead of creating ghost cells, cell pairs that interact over a periodic boundary can also use a periodic correction vector q α α β β . This vector, which can be stored or computed for every cell pair ( C α α , C β β ) , contains the correction which needs to be applied to "wrap" one cell around the domain to neighbour the other. The pairwise distance between two particles p α α ∈ ∈ C α α and p β β ∈ ∈ C β β is then computed as