Our proposal is to employ a local mesh around an atom density. Figure (a2) shows a schematic illustration of a mesh set at a local coordinate system (Scheme 2). The centre of the mesh corresponds with the centre of an atom density. At first, we calculate relative displacements of an atom density λ ′ in this local coordinate system. Then, we calculate the position in the global coordinate system by adding the relative displacement to the original position r, that is, r+λ ′ . By this two-step procedure, a redundant space and the requirement of large grid points in large systems can be avoided. Moreover, we can skip the calculation of equation ( ) every time step. This is because (x 1 , y 1 , z 1 ) always corresponds with the centre of the local mesh and does not change.