It is sometimes useful for the occupied guess orbitals to be other than the lowest ${N}_{\alpha}$ (or ${N}_{\beta}$) 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 $\alpha $ orbitals to be occupied on the first line and $\beta $ 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
or alternatively
$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
or
$occupied 1:4 6 5 7 1:5 7 6 $end
The other $rem variables related to altering the orbital occupancies are:
SCF_GUESS_PRINT
Controls printing of guess MOs, Fock and density matrices.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
0
Do not print guesses.
SAD
1
Atomic density matrices and molecular matrix.
2
Level 1 plus density matrices.
CORE and GWH
1
No extra output.
2
Level 1 plus Fock and density matrices and, MO coefficients and
eigenvalues.
READ
1
No extra output
2
Level 1 plus density matrices, MO coefficients and eigenvalues.
RECOMMENDATION:
None
SCF_GUESS_MIX
Controls mixing of LUMO and HOMO to break symmetry in the initial guess. For
unrestricted jobs, the mixing is performed only for the alpha orbitals.
TYPE:
INTEGER
DEFAULT:
0 (FALSE)
Do not mix HOMO and LUMO in SCF guess.
OPTIONS:
0 (FALSE)
Do not mix HOMO and LUMO in SCF guess.
1 (TRUE)
Add 10% of LUMO to HOMO to break symmetry.
$n$
Add $n\times 10\%$ of LUMO to HOMO ($$).
RECOMMENDATION:
When performing unrestricted calculations on molecules with an even number of
electrons, it is often necessary to break alpha/beta symmetry in the initial
guess with this option, or by specifying input for $occupied.
$molecule 0 1 H 0.000 0.000 0.0 H 0.000 0.000 -10.0 $end $rem METHOD = hf BASIS = 6-31g** UNRESTRICTED = true SCF_ALGORITHM = diis_gdm MAX_DIIS_CYCLES = 1 SCF_GUESS = gwh SCF_GUESS_MIX = 2 $end