(Gaussian only)
For GaussSum, an electron density difference map is what you get when you subtract the electron density before an electronic transition, from the electron density after. This allows visualisation of charge transfer associated with a given transition. This is particularly useful when a transition has a number of contributions from different singly-excited configurations.
Empty the gausssum1.0 directory of any files that don't relate to the current molecule. In particular, you should make sure that there aren't any cube files present that don't relate to the current molecule.
This section uses the Python script EDDM.py. It requires that the Gaussian directory is in the PATH. Please see the Installation section for instructions on adding a directory to the PATH. You can test to see whether the Gaussian directory is already included in the PATH by typing: cubman at the command prompt. If it works, just press CTRL+C to exit.
EDDM.py requires a file UVData.txt in the gausssum1.0 directory. This file is created using UVVis.py as described in the section "How do I get the UV-Vis spectrum of a molecule?".
Place a checkpoint file or formatted checkpoint file for the molecule (e.g. the output of a geometry optimisation) in the folder containing the gausssum1.0 folder. Open the checkpoint file using GaussSum. This will be used to generate cube files.
Choose the script EDDM.py. At this point you should open UVData.txt using Excel or a simple text editor, and choose for which transitions you wish to calculate the electron density difference map.
Enter the transitions into the box in the format shown and click on the GaussSum logo to start.
The steps that EDDM.py takes are as follows (note: all the cube files are stored in the gausssum1.0 directory):
formats the checkpoint file (if necessary) into gausssum1.0/chkpoint.fch
create a coarse cube file for each of the molecular orbitals involved in the transitions you specify. These cube files will remain after the calculation to allow rapid calculation of future electron density difference maps for the same molecule. These are named moXX.cube where XX is the number of the molecular orbital.
create an electron density cube file for each of the relevant molecular orbitals. These cube files will also remain after the calculation for the same reason. However, you can delete them without suffering a large increase in the time taken to create future electron density difference maps. These are named sqXX.cube where XX is the number of the molecular orbital.
for each transition, add the single-excitation contributions (scaled by the % contribution), to create find the electron density before and after the transition. The % contributions are further scaled so that they add to give 100% (this allows better comparison between electron density difference maps).
subtract the electron density before, from the electron density after. This gives the electron density difference map. This is stored in a cube file transXX.cube where XX corresponds to the transition number.