(Experimental) You have the possibility of "freezing" a number of orbitals during a time-propagation. The Hartree and exchange-correlation potential due to these orbitals (which will be the lowest-energy ones) will be added during the propagation, but the orbitals will not be propagated.
Single-active-electron approximation. This option is only valid for time-dependent
calculations (CalculationMode = td). Also, the nuclei should not move.
The idea is that all orbitals except the last one are frozen. The orbitals are to
be read from a previous ground-state calculation. The active orbital is then treated
as independent (whether it contains one electron or two) -- although it will
feel the Hartree and exchange-correlation potentials from the ground-state electronic
It is almost equivalent to setting TDFreezeOrbitals = N-1, where N is the number of orbitals, but not completely.
td/td.F90 : 773
call parse_variable(namespace, 'TDFreezeOrbitals', 0, freeze_orbitals)