How to find the nature of the overlap between different groups of atoms (COOP)
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## How to find the nature of the overlap between different groups of atoms (COOP)

Crystal Orbital Overlap Population (COOP) diagrams were first developed by Huckel. They are especially used in the area of extended solids. They are equivalent to a multiplication of the DOS spectrum and the overlap population (positive or negative) between two groups. In this way, the bond between two groups can be described as bonding or antibonding with respect to each of the molecular orbitals.

Exactly the same information as for creating a PDOS diagram is needed for creating a COOP diagram. That is, you need a log file which is the result of a single-point calculation containing the same keywords as described in "How to find the % contribution of a group to each molecular orbital". You also need a file `Groups.txt` describing the groups of interest in the molecule (see "How to find the % contribution of a group to each molecular orbital"). The rules for `Groups.txt` are not as strict in this case: no atom can be listed twice, but not every atom needs to be listed.

Choose COOP after picking Orbitals. The options for creating a COOP diagram are the same as for creating a DOS or PDOS diagram.

Click on the GaussSum logo to calculate the COOP. Information on the overlap is written to `gausssum3.0/COOP_data.txt` and `gausssum3.0/COOP_spectrum.txt`.

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