  We elucidate the origin of large differences (two-fold or more) in the
fixed-node errors between the first- vs second-row systems for
single-configuration trial wave functions in quantum Monte Carlo calculations.
This significant difference in the fixed-node biases is studied across a set of
atoms, molecules, and also Si, C solid crystals. The analysis is done over
valence isoelectronic systems that share similar correlation energies, bond
patterns, geometries, ground states, and symmetries. We show that the key
features which affect the fixed-node errors are the differences in electron
density and the degree of node nonlinearity. The findings reveal how the
accuracy of the quantum Monte Carlo varies across a variety of systems, provide
new perspectives on the origins of the fixed-node biases in electronic
structure calculations of molecular and condensed systems, and carry
implications for pseudopotential constructions for heavy elements
