10.3 Orbital Analysis 10.3.2 Orbital Localization 10.4 Density Analysis

10.3.3 Donor–Acceptor Orbital Overlaps

(April 13, 2024)

In donor–acceptor (D–A) materials, the overlap between the donor orbital (HOMO of D) and the acceptor orbital (LUMO of A) has been shown to correlate with excited-state charge-transfer character. This overlap can be computed, taking into account relaxation of the monomer geometries upon formation of the D–A complex, using the $rem variable MO_OVERLAPS_TWO_GEOMS, as shown in Example 10.3.3 below. This requires two separate SCF calculations (one for either DD or A in isolation, and another for the dimer D–A), and both calculations must use the same basis set. Therefore, ghost atoms should be added in the monomer calculation so that both SCF calculations use the dimer basis set. Output is delivered in a directory OutputName.MO-overlaps (where OutputName is the name of the Q-Chem output file). This output consists of the $\alpha$ -spin MO coefficients for both systems (CA-Geom1.txt and CA-Geom2.txt), the AO overlap matrix between the two calculations (AO-overlap-TwoGeoms.txt, which need not be an identity matrix because the geometries could differ), and the MO overlap matrix between the two calculations (MO-overlap-TwoGeoms.txt).

Note: Currently, only overlaps between $\alpha$ -spin MOs are considered, hence why only $\alpha$ -spin MO coefficients are returned. This means that this features is implemented only for spin-restricted wave functions.

MO_OVERLAPS_TWO_GEOMS

MO_OVERLAPS_TWO_GEOMS
       Specifies whether to compute molecular orbital overlaps at two different geometries.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Do not compute these overlaps. 1 Used to indicate the first of two required SCF calculations. 2 Used to indicate the second of two required SCF calculations.
RECOMMENDATION:
       The atoms must be ordered in the same way for both calculations. (This is not checked.)

Example 10.8 Compute MO overlaps for a C ${}_{\text{2}}$ H ${}_{\text{4}}\cdots$ C ${}_{\text{2}}$ F ${}_{\text{4}}$ dimer.

$comment
Job 1: this is the dimer
$end

$molecule
0 1
          C      -0.6656405432    -0.0000000000    -3.3601967383
          H      -1.2362909837    -0.9253767714    -3.3623091560
          H      -1.2362909837     0.9253767714    -3.3623091560
          C       0.6656405432     0.0000000000    -3.3601967383
          H       1.2362909837     0.9253767714    -3.3623091560
          H       1.2362909837    -0.9253767714    -3.3623091560
          C      -0.6610251661    -0.0000000000     1.1109504097
          F      -1.3861639022    -1.1014627457     1.1119323203
          F      -1.3861639022     1.1014627457     1.1119323203
          C       0.6610251661    -0.0000000000     1.1109504097
          F       1.3861639022     1.1014627457     1.1119323203
          F       1.3861639022    -1.1014627457     1.1119323203
$end

$rem
method¯¯cam-b3lyp
basis¯¯6-31+G*
mo_overlaps_two_geoms 1 ! for job #1
$end

@@@

$comment
Job 2:  monomer1, with ghost functions for monomer2
$end

$molecule
0 1
          C      -0.6656405432    -0.0000000000    -3.3601967383
          H      -1.2362909837    -0.9253767714    -3.3623091560
          H      -1.2362909837     0.9253767714    -3.3623091560
          C       0.6656405432     0.0000000000    -3.3601967383
          H       1.2362909837     0.9253767714    -3.3623091560
          H       1.2362909837    -0.9253767714    -3.3623091560
         @C      -0.6610251661    -0.0000000000     1.1109504097
         @F      -1.3861639022    -1.1014627457     1.1119323203
         @F      -1.3861639022     1.1014627457     1.1119323203
         @C       0.6610251661    -0.0000000000     1.1109504097
         @F       1.3861639022     1.1014627457     1.1119323203
         @F       1.3861639022    -1.1014627457     1.1119323203
$end

$rem
method          cam-b3lyp
basis           6-31+G*
mo_overlaps_two_geoms 2 ! for job #2
$end

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10.3.3 Donor–Acceptor Orbital Overlaps