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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
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.
Frequencies name the structure, geometries face experiment, and barriers need named corrections.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.