First, we should note that we are primarily concerned with the problem of assigning an absolute index to each orbital. It is also possible to assign more than one index per orbital. For instance, we might use the first index to designate the orbital's irreducible representation, and the second index to designate a particular orbital within that representation. This is the format used in PSI's file34. It seems to us that using a single index for each orbital is typically more convenient for post Hartree-Fock calculations, but this is not necessarily the case.