---
title: "Excited States by Calculation and Simulated Electronic Spectra"
description: "Predicting where a molecule absorbs means treating a state that is not the ground state. Time-dependent methods give excitation energies and intensities cheaply, but they fail in known ways, and recog"
canonical: https://lightmysky.com/learn/science/excited-states-by-calculation-and-simulated-electronic-spectra-mt_DsZ3wfCRkX
source: https://lightmysky.com/learn/science/excited-states-by-calculation-and-simulated-electronic-spectra-mt_DsZ3wfCRkX.md
retrieved: 2026-09-12
---

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# Excited States by Calculation and Simulated Electronic Spectra

Predicting where a molecule absorbs means treating a state that is not the ground state. Time-dependent methods give excitation energies and intensities cheaply, but they fail in known ways, and recognising those failures is part of using them.

Subject: Science · Area: Chemistry · Ages 23 to 24
Page: https://lightmysky.com/learn/science/excited-states-by-calculation-and-simulated-electronic-spectra-mt_DsZ3wfCRkX

## Ready when they can

- Reads computed excitation energies and oscillator strengths into a predicted spectrum
- Identifies which orbitals a transition involves from the computed contributions
- Names a class of excitation that common functionals describe badly and says why
- Compares a computed band position with an experimental one and accounts for the difference

## Lesson: Reading a computed spectrum

The point here is predicting a spectrum before anyone measures it. You start from a list of computed excitations. Each entry gives an excitation energy and an oscillator strength. The energy sets where the band appears, and the oscillator strength sets how tall it is. You broaden each stick into a peak shape to get the predicted spectrum.

**Example.** Suppose the output lists HOMO to LUMO with a weight of 0.8 and two small weights near 0.1. You assign the transition as HOMO to LUMO. You let the largest weight name it and you ignore the small ones.

Some excitations come out badly with common functionals. Charge transfer states are the classic case, and they usually come out too low. The cause sits in the method: the approximate exchange fades too fast with distance, so it undervalues the cost of moving charge far. When you see distant charge shift with too low energy, you name this failure.

You always check a computed band against a measured one. Expect a shift from the solvent, from vibrations, and from method error. You assign the shift to a cause before you trust the number. A bare match with no check means little.

**Recap.** You turn energies and strengths into a spectrum, name each transition by its largest contribution, watch for charge transfer error, and check every band against experiment.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Electron Correlation and Methods Past Hartree-Fock](https://lightmysky.com/learn/science/electron-correlation-and-methods-past-hartree-fock-mt_3F5B8V7CGb)
- [Density Functional Theory and Choosing a Functional](https://lightmysky.com/learn/science/density-functional-theory-and-choosing-a-functional-mt_mmuzLeA62V)
- [Electronic Spectra, Franck-Condon and the Origin of Colour](https://lightmysky.com/learn/science/electronic-spectra-franck-condon-and-the-origin-of-colour-mt_y1hKy9nvAY)

## Opens up

- [Putting the Solvent Into a Calculation](https://lightmysky.com/learn/science/putting-the-solvent-into-a-calculation-mt_3SCFfcvoei)