Similar expressions can be written for the second derivative with respect to the y and z coordinates.
In the finite difference method, the computational domain is discretized using a 3D rectangular gridas shown on the figure below. The grid spacing along the three principal coordinate axes is denoted by Δx, Δy and Δz, respectively. <table><tr><td>[[Image:FD grid.png|thumb|left|480px| The 3D rectangular grid used to mesh the computational domain.]]</td></tr></table> The potential at any point (x,y,z) in the space can be expressed in terms of the potentials at six neighboring grid points along the principal axes. The creates a 7-point computational molecule shown in the figure below:
<table>