Generate reference determinant for UEG.
Generate reference determinant for UEG.

UEG has single-particle energies, so can use the same process as for
molecules and hubbard momentum-space systems.

hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande