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---
# Yee Lattice
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In order to discretize Maxwell's equations with [second-order accuracy](https://en.wikipedia.org/wiki/Finite_difference_method#Accuracy_and_order) for homogeneous regions where there are no discontinuous material boundaries, FDTD methods *store different field components for different grid locations*. This discretization is known as a **Yee lattice**.
The form of the Yee lattice in 3d is shown in the schematic above for a single cubic grid voxel with dimensions $\Delta x \times \Delta x \times \Delta x$. The three components of $\mathbf{E}$ are stored on the *edges* of the cube in the corresponding directions, while the components of $\mathbf{H}$ are stored on the cube *faces*.
More precisely, let a coordinate $(i,j,k)$ in the grid correspond to:
$$\mathbf{x} = (i \hat{\mathbf{e}}_1 + j \hat{\mathbf{e}}_2 + k \hat{\mathbf{e}}_3) \Delta x$$
where $\hat{\mathbf{e}}_k$ denotes the unit vector in the *k*-th coordinate direction. Then, the $\ell$<sup>th</sup> component of $\mathbf{E}$ or $\mathbf{D}$ (or $\mathbf{P}$) is stored for the locations:
$$(i,j,k)+ \frac{1}{2} \hat{\mathbf{e}}_\ell \Delta x$$
The $\ell$<sup>th</sup> component of $\mathbf{H}$, on the other hand, is stored for the locations:
$$(i+\frac{1}{2},j+\frac{1}{2},k+\frac{1}{2})-\frac{1}{2} \hat{\mathbf{e}}_\ell \Delta x$$
In two dimensions, the arrangement is similar except that we set $\hat{\mathbf{e}}_3=0$. The 2d Yee lattice for the $H_z$-polarization ($\mathbf{E}$ in the $xy$ plane and $\mathbf{H}$ in the $z$ direction) is shown in the figure below.
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The consequence of the Yee lattice is that, whenever you need to access field components, e.g. to find the energy density $(\mathbf{E}^* \cdot \mathbf{D} + |\mathbf{H}|^2)/2$ or the flux $\textrm{Re}\, \mathbf{E}^* \times \mathbf{H}$, then the components need to be **interpolated** to some common point in order to remain second-order accurate. Meep automatically does this [interpolation](Introduction.md#the-illusion-of-continuity) for you wherever necessary — in particular, whenever you compute [energy density](Python_User_Interface.md#energy-density-spectra) or [Poynting flux](Python_User_Interface.md#flux-spectra), or whenever you [output a field to a file](Python_User_Interface.md#output-functions), it is stored at the centers of each grid voxel: $(i+0.5,j+0.5,k+0.5)$.
In a Meep simulation, the coordinates of the Yee lattice can be obtained using a [field function](Field_Functions.md#coordinates-of-the-yee-grid).
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