Computed Geometries, Frequencies and Reaction Barriers · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Checking a computed mechanism

Science · Chemistry · ages 20-22
Name ______________________   Date ____________

  1. An optimized structure at a genuine stationary point shows exactly one imaginary frequency. What does that mode establish?

    • It proves the intended reactants and products connect
    • It shows an unstable normal mode, with energy curving downward along that coordinate
    • It shows a broken calculation

  2. An optimized structure at a converged stationary point has strictly positive frequencies for every nontrivial vibrational mode, apart from translations and rotations. What is it?

    • A minimum
    • A transition state
    • A broken calculation

  3. Reactants have energy -100.0 and a zero-point correction of 0.5. The transition state has energy -99.0 and a zero-point correction of 0.4. What is the corrected barrier, in the same energy units?

    • 1.0
    • 1.1
    • 0.9

  4. A bond is off by 0.15 angstrom. What do you conclude?

    • The method or structure needs review
    • The gap is within the usual hundredth-scale guide
    • The experiment must be repeated at once

  5. For the same bond type, a computed bond differs from experiment by 0.01 angstrom, and that accuracy is enough for the job. What do you say?

    • The method failed completely
    • The structure must be re-optimised at once
    • The agreement is fine

  6. A single-point energy on a poor geometry is worth reporting as the energy of the intended optimized structure.

    Circle one:   True   False

  7. A quoted barrier should state which corrections were applied.

    Circle one:   True   False

  8. An optimised structure has one imaginary frequency. What is it, and what confirms it links the right minima?

    • A transition state, confirmed by counting the atoms
    • A transition state, confirmed by following the path down both sides
    • A transition state, confirmed by checking one downhill side only
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Checking a computed mechanism W1-mt_pdiFjK2wPe-s1

  1. It shows an unstable normal mode, with energy curving downward along that coordinate · One imaginary mode at a genuine stationary point shows negative curvature along that coordinate. It does not by itself identify the intended bond motion or prove which minima connect; inspect the motion and follow the downhill path to check that.
  2. A minimum · For a converged stationary point, strictly positive frequencies for every nontrivial vibrational mode, apart from translations and rotations, mark a valley bottom.
  3. 0.9 · Corrected top is -98.6 and corrected reactants are -99.5, so the barrier is 0.9.
  4. The method or structure needs review · A tenth-scale gap usually warns that the method or structure needs review.
  5. The agreement is fine · A hundredth-scale gap is often fine when that accuracy is enough for the job.
  6. False · Good energies on wrong coordinates cannot support that claim.
  7. True · Without stated corrections the number cannot be judged.
  8. A transition state, confirmed by following the path down both sides · One imaginary mode marks the top, and the downhill path must reach both minima.
Worksheet · LightMySky