Harmonic spring approximation

The Harmonic spring (also known as the Hookean spring) approximation assumes that the spring obeys 'Hookes Law, i.e.

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\mathbf F} = -k {\mathbf x}}

where F is the force, x is the extension of the spring, and k is the spring constant.

This approximation is often used for chemical-bond stretching terms in various force fields:

$\Phi _{str}(r_{12})={\frac {1}{2}}K_{str}(r_{12}-b_{0})^{2}$