  We study lithium systems over a range of number of atoms, e.g., atomic anion,
dimer, metallic cluster, and body-centered cubic crystal by the diffusion Monte
Carlo method. The calculations include both core and valence electrons in order
to avoid any possible impact by pseudo potentials. The focus of the study is
the fixed-node errors, and for that purpose we test several orbital sets in
order to provide the most accurate nodal hyper surfaces. We compare our results
to other high accuracy calculations wherever available and to experimental
results so as to quantify the the fixed-node errors. The results for these Li
systems show that fixed-node quantum Monte Carlo achieves remarkably high
accuracy total energies and recovers 97-99 % of the correlation energy.

Computational Physics;Materials Science;Chemical Physics