# softmatter:Harmonic Spring

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Polymeric molecules, such as surfactants and block copolymers, are often treated as bead spring chains where beads of size σ are permanently connected via hormic springs,

$U_{ij}^{harmonic}(r) = \frac{1}{2}k(r-R_{0})^2$

where r is the radial separation between bonded beads, k is the spring constant, and R0 is the equilibrium separation between two beads.

Another commonly used spring in polymer simulation is the FENE spring .