Correlated sampling in quantum Monte Carlo: A route to forces

In order to find the equilibrium geometries of molecules and solids and to perform ab initio molecular dynamics, it is necessary to calculate the forc es on the nuclei. We present a correlated sampling method to efficiently ca lculate numerical forces and potential energy surfaces in diffusion Monte C arlo. This method employs a coordinate transformation, earlier used in vari ational Monte Carlo, to greatly reduce the statistical error. Results are p resented for first-row diatomic molecules.