Excitons in small hydrogenated Si clusters

The excitonic states of four small hydrogenated Si clusters $({\mathrm{SiH}}_4,{\mathrm{Si}}_2{\mathrm{H}}_6,{\mathrm{Si}}_5{\mathrm{H}}_{12},$ and ${\mathrm{Si}}_{10}{\mathrm{H}}_{16})$ are studied using the diffusion quantum Monte Carlo approach. The importance of using accurate guiding wave functions is stressed and we show that the quantum chemical singles-only configuration interaction method and the time-dependent density functional theory within the adiabatic local-density approximation can provide suitable zeroth-order approximations in these systems.