= At-rest force for a symmetric phi-four pair
{title2=$F(c)=8\tanh^2c(1-\tanh c)^2\sim32e^{-4c}$}
For the initial field $\phi(x)=\tanh(x+c)-\tanh(x-c)-1$ and zero initial time derivative, evaluate the <scalar-field momentum flux> at the midpoint: $\phi_x(0)=0$ and $\phi(0)=2\tanh c-1$. The force on the left <kink> is the displayed positive value, attractive toward the right <antikink>. This exact initial expression has the large-separation asymptotic $32e^{-2d}$ for $d=2c$. If the initial velocity at the cut is $\psi(0)$, subtract $\psi(0)^2/2$.
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