1. With INTCOORD or INTCOORD+SYMCOORD files in place, run a calculation with ANHARM=FULLQUARTIC.
2. Create a file titled "NEWMASS" that lists the masses of the atoms for the new isotopomer (one mass for each line of the file).
3. Run xisotrans. You should see that the following files have been created: cartcoord.external, coriolis.external, cubic.external, didQ.external, dipolex.external, DIPOL.external, dipoley.external, dipolez.external, normcoord.external, quadratic.external, quartic.external, and rota.external. These files are (within computational accuracy) identical to the files that would have been created in an ANHARM=FULLQUARTIC calculation with an ISOMASS file identical to the NEWMASS file used by xisotrans.
4. Run xcubic. The output gives the ANHARM=FULLQUARTIC calculated properties for the new isotopomer.