  We investigate the energy dependence of a single-channel effective potential
between the $K^-$ and the "$pp$"-core nucleus, which can be obtained as an
$K^-$-"$pp$" equivalent local potential from a coupled-channel model for
$\bar{K}(NN)$-$\pi(\Sigma N)$ systems. It turns out that the imaginary part of
the resultant potential near the $\pi \Sigma N$ decay threshold can well
approximate the phase space suppression factor of $K^-pp \to \pi \Sigma N$
decay modes. The effects on the pole position of the $\pi(\Sigma N)$ state in
the $\pi \Sigma N$ channel are also discussed.
