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# 4.4.4 Modifying the Occupied Molecular Orbitals

(February 4, 2022)

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 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 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 ($0). RECOMMENDATION: When performing unrestricted calculations on molecules with an even number of electrons, it is often necessary to break alpha/beta symmetry in the initial guess with this option, or by specifying input for$occupied.

Example 4.6  Input for an unrestricted HF calculation on H${}_{2}$ 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


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