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 250 newtons per metre stretched by 0.10 metres pushes back with 25 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.
The rewritten readings were written for this dataset.
More to read
Nonbonded forces in molecular models
Besides their internal bonds, molecules experience attractions to other molecules. Dispersion produces weak, changing attractions between all atoms, including gaseous argon. Dipole-dipole attraction draws a molecule's partially positive end toward another's partially negative end. Hydrogen bonded to nitrogen, oxygen or fluorine can attract a lone pair on a neighboring molecule, forming a hydrogen bond. This can be stronger than the other attractions, but there is no universal strength ranking.
Force fields omit explicit electrons. A van der Waals term models dispersion attraction at moderate separations and steep repulsion when atoms crowd together. Electrostatic terms represent interactions between partial charges. Relating these terms to physical interactions helps you interpret the model.
What bond springs leave out
Quantum bonding involves electrons. In covalent bonds, electron density shared between nuclei holds atoms together. Bond dissociation energy measures the energy needed to separate bonded atoms completely. For ionic bonding, estimates involve ionization energy, electron affinity and Coulomb attraction. Electron exchange symmetry also affects some bonding. A full quantum description accounts for electrons.
Conventional molecular mechanics instead represents atoms and assigns spring-like bond terms with set stiffness. This makes calculations quick and inexpensive. These fixed-connectivity models cannot describe bond breaking because their bonds are predetermined, not merely because they omit explicit electrons.
Predicting and searching molecular shapes
A Lewis diagram is flat; atoms occupy three dimensions. VSEPR predicts arrangements by treating electron groups around an atom as repelling one another. It gives shapes such as bent, trigonal planar and tetrahedral. Molecular polarity depends on bond dipoles and geometry; symmetry can cancel the dipoles.
Force fields assign energies using spring-like and van der Waals terms, among others. Moving atoms toward lower energy relaxes a structure until further changes give no decrease. This usually finds a local minimum. VSEPR is useful for small molecules. For large, flexible molecules, conformational searching compares possible arrangements, often using repeated force-field relaxations.
Bond order and spring stiffness
Sharing one electron pair gives a single covalent bond. Double bonds share two pairs, and triple bonds share three. The extra pairs form pi bonds, generally weaker than the sigma bond formed by the first pair.
For comparable bonds between the same elements, more shared pairs usually shorten the bond and increase stiffness. Force fields encode this with different spring constants: double bonds generally get larger values than singles, and triples larger still. Each bond type also has an equilibrium length. A bond-stretch entry identifies the bond, its preferred length and its resistance to stretching.
2 · Watch
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
13 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.