Nonbonded interactions
This page is quite redundant to pages "intercrystolecular forces" and "nonbonded forces".
This page is maybe more about forces than interactions. Rename?
Then again it covers phenomenological molecular dynamic models that are not exactly fundamental forces. Split?
(wiki-TODO: Page might need some review and cleanup.)
The forces
The attractive forces
(The Van der Waals force which split up into three):
- London (dispersion) force (mutually induced dipole force),
- Debye force (dipole - induced dipole force)
- Keesom force (dipole - dipole force)
Characteristics:
- always attractive
- relatively weak (except exchange interaction but this is not considered non-bonded but covalently bonded)
- longer range => many atoms can contribute => forces add up
Since attractive forces add up but repulsive ones do not the bigger contacting surface areas get the smaller equilibrium separations get, (down to some point).
Nanosystems: The term "Van der Waals forces" is usually used for the attractive components alone by physicists.
(wiki-TODO: The wikipedia page about superlubricity (here: [1] 2018-08) mentions repulsive VdW forces (negative Hamaker constant). Find out if that is just due to a mix-in of overlap repulsion (likely?) or a genuine effect?)
Casimir forces can be described as relativistically retared vdW forces supposedly.
(wiki-TODO: Get more clear on that.)
The odd one out:
Atractive exchange interaction force
… more spacial freedom for the electron makes for smaller impulse by the Heisenberg see saw.
Usually not considered non-bonded but covalent or related to covalent.
The repulsive forces
The following are more or less just different names for the same thing:
- overlap repulsion a.k.a.
- steric repulsion (a.k.a. hard-core, Born)
- Pauli repulsion
- repulsive exchange interaction force a.k.a.
- degeneracy pressure (not a directed force but isotropic pressure as a consequence in large ensembles)
Characteristics:
- always repulsive
- can get very strong
- short range, exponential decay over distance (approximately) => only nearby atoms contribute
The odd one out:
Exotic repulsive Keesom force
Real electric dipoles need to be forced in opposing direction by motion guides.
Models
According to Nanosystems 3.3.2.e. :
In computational chemistry it is common that polar interactions are treated separately but overlap repulsion is included
MM2
exp-6 potential
As specific example in the MM2 model used is the Buckingham (or exp-6) potential.
A rough estimation for pairwise interactions. In MM2 corrected parameters are used to get better results.
E.g. for C to H nonbonded interaction forces.
(wiki-TODO: Add the math of the model & legend.)
Corrections tweaks "hacks"
Atoms in for gem-gum technology relevant materials are strongly bond to other atoms which can more or less significantly shift electron density distributions away from high symmetry. This is not captured by the simple model and thus calls for corrections.
- In case of the electron density shifts in the nonbonded interaction between nitrogen oxygen (both sp3 – their lone pairs are contacting) this is solved by the introduction of lone pair pseudoatoms for calculations. (wiki-TODO: What does that mean exactly?))
- Covalently surface passivating hydrogen atoms have their electrons move to the passivated surface a bit. This is solved by the hack of shifting the position of the atom inward for calculations. By 0.915 in case of the MM2 model.)
Related
- Van der Waals force
- Superlubricity
- Negative pressure bearings
- Energy, force, and stiffness
- Mass and spring molecular modelling
- Intercrystolecular forces
External links
- Wikipedia: intermolecular force