  In this article, we calculate the contributions of the vacuum condensates up
to dimension-10 in the operator product expansion, and study the
$J^{PC}=0^{++}$, $1^{+-}$, $2^{++}$ $D^*\bar{D}^*$, $D_s^*\bar{D}_s^*$,
$B^*\bar{B}^*$, $B_s^*\bar{B}_s^*$ molecular states with the QCD sum rules. In
calculations, we use the formula $\mu=\sqrt{M^2_{X/Y/Z}-(2{\mathbb{M}}_Q)^2}$
to determine the energy scales of the QCD spectral densities. The numerical
results favor assigning the $Z_c(4020)$ and $Z_c(4025)$ as the $J^{PC}=0^{++}$,
$1^{+-}$ or $2^{++}$ $D^*\bar{D}^*$ molecular states, the $Y(4140)$ as the
$J^{PC}=0^{++}$ $D^*_s{D}_s^*$ molecular state, the $Z_b(10650)$ as the
$J^{PC}=1^{+-}$ $B^*\bar{B}^*$ molecular state, and disfavor assigning the
$Y(3940)$ as the ($J^{PC}=0^{++}$) molecular state. The present predictions can
be confronted to the experimental data in the futures.
