It is sometimes useful for the occupied guess orbitals to be other than the lowest (or ) orbitals. Reasons why one may need to do this include:
To converge to a state of different symmetry or orbital occupation.
To break spatial symmetry.
To break spin symmetry, as in unrestricted calculations on molecules with an even number of electrons.
There are two mechanisms for modifying a set of guess orbitals: either by SCF_GUESS_MIX, or by specifying the orbitals to occupy. Q-Chem users may define the occupied guess orbitals using the $occupied or $swap_occupied_virtual keywords. In the former, occupied guess orbitals are defined by listing the orbitals to be occupied on the first line and on the second. In the former, only pair of orbitals that needs to be swapped is specified.
Note: (1) To prevent Q-Chem to change orbital occupation during SCF procedure, MOM_START option is often used in combination with $occupied or $swap_occupied_virtual keywords. (2) The need for orbitals renders these options incompatible with the SAD guess. Most often, they are used with SCF_GUESS = READ.
The format for modifying occupied guess orbitals is as follows:
$occupied 1 2 3 4 ... NAlpha 1 2 3 4 ... NBeta $end
$swap_occupied_virtual <spin> <io1> <iv1> <spin> <io2> <iv2> $end
Occupied and virtual orbitals can also be swapped
$swap_occupied_virtual alpha 5 6 beta 6 7 $end
This is identical to
$occupied 1 2 3 4 6 5 7 1 2 3 4 5 7 6 $end
$occupied 1:4 6 5 7 1:5 7 6 $end
The other $rem variables related to altering the orbital occupancies are:
$molecule 0 1 H 0.000 0.000 0.0 H 0.000 0.000 -10.0 $end $rem UNRESTRICTED = true METHOD = hf BASIS = 6-31g** SCF_ALGORITHM = diis_gdm MAX_DIIS_CYCLES = 1 SCF_GUESS = gwh SCF_GUESS_MIX = 2 $end