= Worm-like chain force-extension interpolation
{title2=$f=\frac{k_BT}{L_p}\{[4(1-x/L)^2]^{-1}-1/4+x/L\}$}
The interpolation links chain extension to entropic tensile <force>, with contour length $L$ and <persistence length> $L_p$. Small extension gives spring constant $3k_BT/(2L_pL)$; near $x=L$ the force stiffens as $(1-x/L)^{-2}$. The divergence assumes an inextensible backbone and is not a physical infinite tensile force. A branch fit estimates available contour and flexibility, not an unfolding activation barrier by itself.
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