  We present high-accuracy correlated calculations of small Si$_x$H$_y$
molecular systems both in the ground and excited states. We employ quantum
Monte Carlo (QMC) together with a variety of many-body wave function approaches
based on basis set expansions. The calculations are carried out in a
valence-only framework using recently derived correlation consistent effective
core potentials. Our primary goal is to understand the fixed-node diffusion QMC
errors in both the ground and excited states with single-reference trial wave
functions. Using a combination of methods, we demonstrate the very high
accuracy of the QMC atomization energies being within $\approx$ 0.07 eV or
better when compared with essentially exact results. By employing proper
choices for trial wave functions, we have found that the fixed-node QMC biases
for total energies are remarkably uniform ranging between $1-3.5$ % with
absolute values at most $\approx$ 0.2 eV across the systems and several types
of excitations such as singlets and triplets as well as low-lying and
Rydberg-like states. Our results further corroborate that Si systems, and
presumably also related main group IV and V elements of the periodic table (Ge,
Sn, etc), exhibit some of the lowest fixed-node biases found in valence-only
electronic structure QMC calculations.
