The electron density difference map (EDDM) [Gaussian only]
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## The electron density difference map (EDDM) [Gaussian only]

An EDDM is a representation of the changes in electron density that occur for a given electronic transition. It is calculated using the information on the single-excited configurations that contribute to each transition. The relative contribution is based on the square of the configuration's coefficient. Note that for some programs (e.g. Gaussian) these squares are not guaranteed to sum to 1.0 and so should not be regarded as scientifically accurate. However, they should be sufficient for the purposes of generating a diagram.

After you have plotted the UV-Vis or circular dichroism spectrum, copy a checkpoint file (or formatted checkpoint file) into the gausssum3.0 directory. Then select "Electronic transitions" and choose the "Create EDDM script?" option. Click on the GaussSum logo to convolute the spectrum and generate `gausssum3.0/eddm.bat` (`gausssum3.0/eddm.sh` on Linux).

Next you need to set the environment variable G03DIR to the directory containing the Gaussian binaries. To generate the EDDM, at a command prompt run "eddm.bat N", where N is the transition number (starting from 1 for the lowest energy transition).

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