The approach used for these models is described in Nulsen et
al. (2010, ApJ, 722, 55). 

First make the XSPEC local model in the model directory (see USING and
README files there), then make the support routine in the support
directory.  You may need to edit the Makefile in the support directory
to get it to work.

After running make successfully in the support directory, in the xas
directory, create the following soft links: 
ln -s support/cosinfo support/Basic_Constants support/stdcosmo.pars .

Start XSPEC and run the script nfwmass.xcm to load spectra and setup
the model.

The mass models can fail if the parameters are unreasonable.  Try
setting initial temperatures to help the model along.

Once a reasonable fit has been found with the abundances tied, try
freeing them.  Note that abundances for the outer shells may need to
be grouped to keep them reasonable.  XSPEC allows algebraic
expressions for constraints, so, for example, you can use (integers
refer to parameters): 
newpar 258 = 0.5 * (229 + 287)

How would you determine M_2500, M_500, M_200, etc?

How would you determine confidence ranges for these?

For the impatient, Zbetanfw.xcm sets parameters to the best fit found
so far.