  Most theoretical investigations about titanium oxide clusters focus on
(TiO$_2$)$_n$. However, many Ti$_n$O$_m$ clusters with $m\neq 2n$ are produced
experimentally. In this work, first-principles calculations are performed to
probe the evolution of Ti$_n$O$_m$ clusters. Our investigations show that for
$n=3$-$11$, there exist one relatively stable specie; while for $n=12$-$18$,
there are two relatively stable species: Ti-rich and O-rich species. HOMO-LOMO
calculations show that the gap can be tuned by changing the size and
configurations of Ti$_n$O$_m$ clusters. Our investigation provides insights
into the evolution of cluster-to-bulk process in titanium oxide.
