  We present electronic and magnetic properties of a honeycomb compound
In$_{3}$Cu$_{2}$VO$_{9}$ in this paper. We find that the parent phase is a
charge transfer insulator with an energy gap of about 1.5 eV. Singly occupied
d$_{3z^{2}-r^{2}}$ electrons of copper ions contribute an $S$ = 1/2 spin, while
vanadium ions show nonmagnetism. Oxygen 2$p$ orbitals hybridizing with a small
fraction of Cu 3$d$ orbitals dominate the density of states near $E_{F}$. The
planar nearest-neighbor, next-nearest-neighbor and interplane superexchange
couplings of Cu spins are $J_{1}$ $\approx$ 16.2 meV, $J_{2}$ $\approx$ 0.3 meV
and $J_{z}$ $\approx$ 1.2 meV, suggesting a low-dimensional antiferromagnet
\cite{Sondhi10}. We propose that the magnetic frustration along the c-axis
leads to a quantum spin disorder in In$_{3}$Cu$_{2}$VO$_{9}$, in accordance
with the recent experiments. {abstract}
