  We investigated systematically whether the S-wave ($\bar{Q}q$) meson and the
($Q\bar{q}$) meson may form S-wave bound states in a chiral SU(3) quark model
by solving the resonating group method equation. Here $Q=c$ or $b$ and $q=u$,
$d$ or $s$. Our preliminary calculation disfavors the existence of $I=\frac12$
($\bar{Q}l$)-($Q\bar{s}$) molecules ($l=u,d$) while favors the existence of
isoscalar $B\bar{B}$, $B^*\bar{B}^*$ (J=2) and $B\bar{B}^*$ (C=+) molecules.
The existence of isovector (charm-anticharm) and (charm-bottom) molecules is
also disfavored. Therefore the resonance-like structure $Z^+(4051)$ is unlikely
to be an S-wave $D^*\bar{D}^*$ molecule.
