Chapter 7. How do I get the UV-Vis or circular dichroism spectrum of a molecule?
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# Chapter 7. How do I get the UV-Vis or circular dichroism spectrum of a molecule?

## The UV-visible spectrum

Open a log file that is the result of a TD-DFT calculation. For best results, a Gaussian TD-DFT calculation should include the following keyword "IOP(9/40=2)".

Choose the Electronic transitions from the list of operations on the left.

Choose the start and end (in nm) of the convoluted spectrum, as well as the number of points you wish to have in the calculated spectrum. FWHM refers to the full width at half-maximum of the gaussian curves used to convolute the spectrum. FWHM should be entered in cm-1.

After you have set the various parameters, click on the GaussSum logo to run convolute the spectrum.

The details are written to `gausssum3.0/UVSpectrum.txt` and `gausssum3.0/UVData.txt`. The file `UVData.txt` contains information on the contribution of singly-excited configurations to each electronic transition.

If there is a file in the `gausssum3.0` directory called `orbital_data.txt` containing information on the percent contributions of various groups (e.g. ligands and metal centers) to the various molecular orbitals, GaussSum will use that data. It will calculate, for each transition, the change in charge density on each group. This information will be added to `gausssum3.0/UVData.txt`. (For more information, please see "How to find the % contribution of a group to each molecular orbital".)

For information on the formula used to convolute the UV-Vis spectrum, please see this pdf.

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How to find the nature of the overlap between different groups of atoms (COOP) | Home | The circular dichroism spectrum