New in version 2.2 is the ability to have the gas density variable as a power law in radius, i.e. , where and are the density and radius at the cloud illuminated face. NB Note that this creates the possibility for calculations which do not end. This because the citeria for a model to end are either that the input column density is reached, or that the electron fraction falls below xeemin. If radexp has a value -1 then the column density integral converges only logarithmically at best, and the specified column may never be reached. If radexp -2 the local ionization parameter will increase with radius, and so the gas will not recombine and the xeemin criterion will not be met.