= Noncovalent interaction
A <noncovalent interaction> is an attraction or repulsion between chemical groups that does not require forming a new ordinary <covalent bond> between them. In <biochemistry>, examples include <hydrogen bonds>, <electrostatic interactions> and <Van der Waals forces>, acting either between molecules or within one molecule. These interactions help stabilize biomolecular structures and enable <molecular recognition>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[entry 09875, non-covalent interactions in biomolecules]{ref}
The <Lennard-Jones potential> gives a simple distance-dependent example:
$$
U(r)=4\epsilon\left[(\sigma/r)^{12}-(\sigma/r)^6\right].
$$
The steep positive term models short-range repulsion; the negative term supplies attraction. Here $r$ is separation, $\epsilon$ sets the energy scale and $\sigma$ sets a length scale.<Lennard-Jones Fluid Reference Calculations: Cuboid Cell by NIST>[section 3A, traditional pair potential]{ref}
Substitution gives $U(\sigma)=0$. At $r=2^{1/6}\sigma$, the sixth-power term is $1/2$ and the twelfth-power term is $1/4$, so $U=-\epsilon$. A pair can therefore have a preferred separation without acquiring a new covalent bond. Bringing the groups still closer eventually raises the energy sharply.
This is an illustrative pair model, not a complete description of every hydrogen bond or protein-binding event. Real molecular association also depends on geometry, other contacts and the surrounding <solvent>. Its central lesson is that noncovalent does not mean energetically negligible or independent of distance.
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