Cluster shape

There are two different options for the cluster shape, which affect which atoms are included in the cluster when layers and surfaces are defined in the cluster definition box. Namely:

• Parabolic. The cluster ends in a parabolic surface, which is axially symmetric with respect to the z axis(i.e., the surface normal) and has the reference point (x₀,y₀,z₀) as its focus. The surface is defined as the points for which the sum of distances to the surface and to the reference point is a constant that equals d_max. It is convenient to choose the reference point at the position of the emitter, since in photoemission, when the photoelectron suffers an scattering event at an atom located on the cluster boundary, the electron path is at least the distance from the emitter to the cluster boundary plus the distance from the boundary to the surface.
• Spherical. The cluster ends in a sphere centered at the reference point (x₀,y₀,z₀) and whose radius is d_max.

See the following figure for a graphical representation. Yellow atoms are those included in the cluster, white atoms are those that are left out, and the green background represents the region occupied by the solid.