A full configuration interaction quantum Monte Carlo study of ScO, TiO and VO molecules

Abstract: 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.