Latest revision as of 10:43, 11 February 2024

The forces

The repulsive forces:

• overlap repulsion a.k.a. exchange force a.k.a. steric repulsion (a.k.a. hard-core,Born) ... (pauli repulsion, degeneracy pressure?)

Characteristics:

• always repulsive
• can get very strong
• sort range, exponetial decay (approximately) => only nearby atoms contribute

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
• 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) mentiones 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?)

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

• Superlubricity
• Negative pressure bearings
• Energy, force, and stiffness
• Mass and spring molecular modelling
• Intercrystolecular forces

External links

• Wikipedia: intermolecular force