Your first choice as a user is the type of data file to use. There are two types of data files: ATOMIC and LEDCOP. The ATOMIC files are the latest data files produced by the LANL opacity team (groups T-1 and XCP-5) and are the preferred files.
See file of currently available materials for the current list of available materials. The material ID numbers are prefixed by the letter n. The LEDCOP files are older cross section files and are included for the sake of continuity.
The default is to use the most recent available data file for each element.
The default is to specify mixtures by number fraction and use the naturally occurring isotopic weights.
The default is to use temperatures in the range of 0.1 to 10 keV.
One should exercise some restraint in choosing the total number of temperature and density points, especially for mixtures containing a large number of elements. The run times can become quite long. The Web Page will give you an time estimate on the Output Selection page after you submit your calculation from the first input page. If the time estimate is too long, the Web Page will refuse the calculation.
One point to be noted is that the range of densities for which cross sections are tabulated varies with temperature. At low temperatures there is a relatively low upper bound on density. This causes the TOPS code to attempt to find opacities for which cross section data do not exist. If this happens, the TOPS code prints a warning message on both the columnar table output and on all graphs except the 3-D graphs. It then uses the cross sections for the highest density point for that temperature to calculate the gray opacities. If multigroup opacities are requested, that gray opacity is put into each multigroup spot so that there is no variation of opacity with photon energy. This should point out that the displayed multigroup opacity does not represent a true multigroup calculation for this point. The frequency dependent data is handled the same way as the multigroups.
Just as there is a pratical limit to the product of number of temperatures times number of densities, there is a limit on the product of number of temperatures times number of densities times number of group boundaries. Currently this product is limited to 50000. Thus if you choose the default temperature grid with 50 temperatures and a density grid with 20 densities, you can still have 50 photon groups.
Special note to SESAME users. This Web Page is now setup to output the special SESAME format ASCII files for opacities. Users must have already obtained the SESAME codes from T-1 at LANL before they can use these tables. See the SESAME writeup for more details about the tables produced by this Web Page.