  Accurate ab initio calculations of 3d transition metal monoxide molecules
have attracted extensive attention because of its relevance in physical and
chemical science, as well as theoretical challenges in treating strong electron
correlation. Meanwhile, recent years have witnessed the rapid development of
full configuration interaction quantum Monte Carlo (FCIQMC) method to tackle
electron correlation. In this study, we carry out FCIQMC simulations to ScO,
TiO and VO molecules and obtain accurate descriptions of 13 low-lying
electronic states (ScO $^2\Sigma^+$, $^2\Delta$, $^2\Pi$; TiO $^3\Delta$,
$^1\Delta$, $^1\Sigma^+$, $^3\Pi$, $^3\Phi$; VO $^4\Sigma^-$, $^4\Phi$,
$^4\Pi$, $^2\Gamma$, $^2\Delta$), including states that have significant
multi-configurational character. The FCIQMC results are used to assess the
performance of several other wave function theory and density functional theory
methods. Our study highlights the challenging nature of electronic structure of
transition metal oxides and demonstrates FCIQMC as a promising technique going
forward to treat more complex transition metal oxide molecules and materials.
