What a learner can do afterwards
- Names the terms of a force field and says what each is standing in for
- Explains why a force field cannot model bond breaking and what that rules out
- Describes a conformational search and explains why one minimisation is not enough
- Judges whether a force field is appropriate for a stated problem and gives the reason
1 · Read
A force field models a molecule with zero electrons. Bonds become springs that push back when stretched, angles become hinges with preferred openings, torsions become dials with favoured twists, and distant atoms attract or repel softly. Each term stands in for real electron effects with a fitted number: a double bond gets a bigger spring constant than a single one because it resists stretching harder.
The spring math is ordinary Hooke's law. Force equals k times x: a bond with k of 200 newtons per metre stretched by 0.15 metres pushes back with 30 newtons. Stored energy is half k times x squared: a bond with k of 400 and stretch of 0.2 holds half times 400 times 0.04, which is 8 joules. Same springs, two questions: how hard it pulls, and how much energy it stores.
Springs cannot snap, and that draws the hard boundary. A force field can never model bond breaking, so reactions, transition states, and anything where electrons rearrange are ruled out. Judge every job against that line: shapes and conformers are in, bond making and breaking are out.
Use the cheapness to search, not to settle. One minimisation slides into the nearest dip, which may be a mere side valley, so launch many starts and keep the lowest result. A flexible ten-atom chain is the perfect customer: thousands of shapes searched with springs, while the one reaction step goes to a method with electrons.
Springs find shapes fast and cheap, but they never snap, so reactions belong elsewhere.
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.