  The structure of the tetraquark candidate $Z_{c}(3900)$, which was
experimentally reported in $e^+ e^-$ collisions, is studied by the s-wave
meson-meson coupled-channel scattering on the lattice. The s-wave interactions
among the $\pi J/\psi$, $\rho \eta_{c}$ and $D \bar{D}^{*}$ channels are
derived from (2+1)-flavor dynamical QCD simulations at $m_{\pi}=410$--$700$
MeV. It is found that the interactions are dominated by the off-diagonal $\pi
J/\psi$-$D \bar{D}^{*}$ and $\rho \eta_{c}$-$D \bar{D}^{*}$ couplings. With the
interactions obtained, the s-wave two-body amplitudes and the pole position in
the $\pi J/\psi$-$\rho \eta_{c}$-$D \bar{D}^{*}$ coupled-channel scattering are
calculated. The results show that the $Z_{c}(3900)$ is not a conventional
resonance but a threshold cusp. A semiphenomenological analysis with the
coupled-channel interaction to the experimentally observed decay mode is also
presented to confirm the conclusion.
