  Fixed-node diffusion Monte Carlo (FNDMC) is a stochastic quantum many-body
method that has a great potential in electronic structure theory. We examine
how FNDMC satisfies exact constraints, linearity and derivative discontinuity
of total energy $E(N)$ vs. fractional electron number $N$, if combined with
mean-field trial wave functions that miss such features. H and Cl atoms with
fractional charge reveal that FNDMC is well able to restore the piecewise
linearity of $E(N)$. The method uses ensemble and projector ingredients to
achieve the correct charge localization. Water-solvated Cl$^-$ complex
illustrates superior performance of FNDMC for charged noncovalent systems.
