  All-electron Fixed-node Diffusion Monte Carlo (FN-DMC) calculations for the
nonrelativistic ground-state energy of the water molecule at equilibrium
geometry are presented. The determinantal part of the trial wavefunction is
obtained from a perturbatively selected Configuration Interaction calculation
(CIPSI method) including up to about 1.4 million of determinants. Calculations
are made using the cc-pCV$n$Z family of basis sets, with $n$=2 to 5. In
contrast with most QMC works no re-optimization of the determinantal part in
presence of a Jastrow is performed. For the largest cc-pCV5Z basis set the
lowest upper bound for the ground-state energy reported so far of -76.43744(18)
is obtained. The fixed-node energy is found to decrease regularly as a function
of the cardinal number $n$ and the Complete Basis Set limit (CBS) associated
with {\it exact nodes} is easily extracted. The resulting energy of
-76.43894(12) -in perfect agreement with the best experimentally derived value-
is the most accurate theoretical estimate reported so far. We emphasize that
employing selected CI nodes of increasing quality in a given family of basis
sets may represent a simple, deterministic, reproducible, and systematic way of
controlling the fixed-node error in DMC.
