4.4 SCF Initial Guess

4.4.3 Modifying the Occupied Molecular Orbitals

(June 30, 2021)

It is sometimes useful for the occupied guess orbitals to be other than the lowest Nα (or Nβ) 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 α orbitals to be occupied on the first line and β 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×10% of LUMO to HOMO (0<n<10).
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.

Example 4.6  Input for an unrestricted HF calculation on H2 in the dissociation limit, showing the use of SCF_GUESS_MIX = 2 (corresponding to 20% of the alpha LUMO mixed with the alpha HOMO). Geometric direct minimization with DIIS is used to converge the SCF, together with MAX_DIIS_CYCLES = 1 (using the default value for MAX_DIIS_CYCLES, the DIIS procedure just oscillates).

$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

View output