The work function is the minimum thermodynamic work needed to remove an electron from a solid (or surface) to a point in the vacuum. The work function is not a characteristic of a bulk material, but rather a property of the surface of the material.

The work function W for a given surface is defined by the difference between Fermi level and electrostatic potential in the vacuum:

$W = -e\phi - E_f$

where $\phi$ is the electrostatic potential in the vacuum and $E_f$ is the Fermi level of the surface.

Those two quantities can be obtained from PDOS and V_HARTREE_CUBE.

One can use Cubecruncher to compute the profile along the normal of the surface to obtained the electrostatic potential of the given surface and vacuum.

cubecruncher.x -i input.cube -o output.cube -1d_profile 3 0.001