you can find the tabulated potential used in the example script here
The header of the file is:
1 6.0 6.0 6.0 0.3 6.0 0.3 0 0 0
The first number is the dimension of your potential, followed by vectors for boxsize, binsize and offset.
The following grid has to be ordered like z-y-x, meaning:
x0 y0 z0 U(r)
x0 y0 z1 U(r)
x0 y0 zn U(r)
x0 y1 z0 U(r)
x0 y1 z1 U(r)
x0 y1 zn U(r)
And so on. In the tcl script, you can load and activate the potential via
external_potential tabulated file "harmonic_potential.dat" scale "1."
'scale' can be set of values, each corresponding to a particle type so you can exclude types or scale them accordingly.
Note that currently only linear interpolation of the tabulated potential is implemented and you need so set
cellsystem domain_decomposition -no_verlet_list.