---
title: "Computed Geometries, Frequencies and Reaction Barriers"
description: "A calculation is run to answer a question, and the answer has to be checked. Optimising a structure, computing its vibrations and locating the point at the top of a barrier are the three jobs that mak"
canonical: https://lightmysky.com/learn/science/computed-geometries-frequencies-and-reaction-barriers-mt_pdiFjK2wPe
source: https://lightmysky.com/learn/science/computed-geometries-frequencies-and-reaction-barriers-mt_pdiFjK2wPe.md
retrieved: 2026-09-12
---

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# Computed Geometries, Frequencies and Reaction Barriers

A calculation is run to answer a question, and the answer has to be checked. Optimising a structure, computing its vibrations and locating the point at the top of a barrier are the three jobs that make a computed mechanism worth quoting.

Subject: Science · Area: Chemistry · Ages 20 to 22
Page: https://lightmysky.com/learn/science/computed-geometries-frequencies-and-reaction-barriers-mt_pdiFjK2wPe

## Ready when they can

- Explains how a frequency calculation confirms that a structure is a minimum or a transition state
- Compares a computed geometry with an experimental one and comments on the size of the difference
- Extracts an activation barrier from computed energies and states which corrections were applied
- Says why a single-point energy on a poor geometry is not worth reporting

## Lesson: Checking a computed mechanism

A frequency calculation checks what a structure is. No imaginary frequency means a minimum, a valley bottom. Exactly one imaginary frequency means a transition state, the top of a pass.

Compare each computed geometry with experiment and judge the gap. A bond off by a hundredth of an angstrom is fine. A bond off by a tenth warns that the method or the structure is wrong.

**Example.** A barrier comes from computed energies with stated corrections. Subtract reactant energy from top-of-barrier energy, then add the same corrections both sides. Quote the barrier only with its corrections named.

**Tip.** Never report a single-point energy on a poor geometry. Optimise with a decent method first, then confirm the transition state links the right two minima before quoting anything.

**Recap.** Frequencies name the structure, geometries face experiment, and barriers need named corrections.

## Practice

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

## Needs first

- [Transition State Theory and the Eyring Equation](https://lightmysky.com/learn/science/transition-state-theory-and-the-eyring-equation-mt_JVjDMcXds1)
- [Density Functional Theory and Choosing a Functional](https://lightmysky.com/learn/science/density-functional-theory-and-choosing-a-functional-mt_mmuzLeA62V)

## Opens up

- [Reaction Dynamics: Trajectories Across a Potential Energy Surface](https://lightmysky.com/learn/science/reaction-dynamics-trajectories-across-a-potential-energy-surface-mt_6ctTUYqYtS)