= Energetic extension of a strongly stretched worm-like chain
{title2=$x/L\simeq1-\tfrac12\sqrt{k_BT/(fL_p)}+f/K_s$}
An inextensible <worm-like chain> gains extension by reducing configurational fluctuations. At high force its fractional entropic extension approaches $1-\tfrac12\sqrt{k_BT/(fL_p)}$. A finite axial stretch modulus $K_s$ adds the leading energetic extension $f/K_s$. This approximation distinguishes suppressed bending fluctuations from actual stretching of the chain backbone; it does not describe an unfolding event that changes contour length.
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