  Diffusion Monte Carlo (DMC) calculations typically yield highly accurate
results in solid-state and quantum-chemical calculations. However, operators
that do not commute with the Hamiltonian are at best sampled correctly up to
second order in the error of the underlying trial wavefunction, once simple
corrections have been applied. This error is of the same order as that for the
energy in variational calculations. Operators that suffer from these problems
include potential energies and the density. This paper presents a new method,
based on the Hellman-Feynman theorem, for the correct DMC sampling of all
operators diagonal in real space. Our method is easy to implement in any
standard DMC code.
