In theoretical photochemistry, we simulate what happens when molecules and materials absorb light. In order to excite electrons in organic materials, a requirement for photovoltaic devices, the light has to be in the ultraviolet range of the electromagnetic spectrum. To characterise molecules, a UV/Vis spectrometer measures how absorption varies with the energy of the incident light. We can simulate this process using calculations, and plot a spectrum, without ever going into the lab. This post shows how to plot one of these spectra from the output of an electronic structure calculation.

What we start with are a list of electronic excitation energies, and their oscillator strengths. The