  We present unbiased, finite--variance estimators of energy derivatives for
real--space diffusion Monte Carlo calculations within the fixed--node
approximation. The derivative $d_\lambda E$ is fully consistent with the
dependence $E(\lambda)$ of the energy computed with the same time step. We
address the issue of the divergent variance of derivatives related to
variations of the nodes of the wave function, both by using a regularization
for wave function parameter gradients recently proposed in variational Monte
Carlo, and by introducing a regularization based on a coordinate
transformation. The essence of the divergent variance problem is distilled into
a particle-in-a-box toy model, where we demonstrate the algorithm.
