1Theory of Condensed Matter Group, Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, UK
2Department of Physics, Imperial College London, London SW7 2AZ, UK
Exact expressions are derived for forces within the mixed and pure fixed-node diffusion quantum Monte Carlo (DMC) methods. These expressions include the "nodal terms" which arise from the discontinuity in the gradient of the DMC wave function at the nodal surface. We devise a practical scheme for estimating the nodal terms, and demonstrate that their inclusion leads to very accurate forces.